Corresponds to NEO/py commit 3efbbfe3 ("master: automatically discard
feeding cells that get out-of-date").

NEO 1.10

* tag 'v1.10': (55 commits)
  Release version 1.10
  Maximize resiliency by taking into account the topology of storage nodes
  storage: also commit updated cell TID at each replicated chunk of 'obj' records
  storage: skip useless work when unlocking transactions
  qa: flush logs at the end of each test when -L is not used
  qa: add a log in case that a mysterious bug happens again
  storage: clarify log about data deletion of discarded cells
  debug: new example to run the profiler for 1 minute
  mysql: fix replication of big oids (> 16M)
  tests/cluster: speedup waiting a bit
  protocol: update packet docstrings
  Bump protocol version
  protocol: a single byte is more than enough to encode enums
  protocol: small cleanup in packet registration
  Optimize resumption of replication by starting from a greater TID
  importer: update comment about a workaround for ZODB3
  Micro-optimization of p64/u64
  qa: add a log in testBackupNodeLost for easier debugging
  Document that the bug when checking replicas may also cause the master to crash
  storage: stop logging 'Abort TXN' for txn that have been locked
  storage: split _migrate2() for reusable _alterTable()
  qa: new testStorageUpgrade
  qa: update testStorageUpgrade data for what is not automatically upgraded
  qa: original data for the future testStorageUpgrade
  sqlite: fix indexes of upgraded db
  importer: fix NameError when recovering during tpc_finish
  fixup! importer: fetch and process the data to import in a separate process
  Serialize empty transaction extension with an empty string
  client: fix partial import from a source storage
  qa: give a title to subprocesses of functional tests
  importer: give a title to the 'import' and 'writeback' subprocesses
  importer: fetch and process the data to import in a separate process
  importer: new option to write back new transactions to the source database
  importer: log when the transaction index for FileStorage DB is built
  importer: open imported zodb in read-only whenever possible
  fixup! mysql: fix remaining places where a server disconnection was not catched
  fixup! storage: speed up replication by sending bigger network packets
  mysql: do not full-scan for duplicates of big oids if deduplication is disabled
  mysql: fix remaining places where a server disconnection was not catched
  fixup! Add support for custom compression levels
  importer: reenable compression by default
  qa: review testImporter
  qa: remove a few uses of 'chr'
  Fix a few issues with ZODB5
  importer: small code cleanup in speedupFileStorageTxnLookup patch
  importer: do not trigger speedupFileStorageTxnLookup uselessly
  Add support for custom compression levels
  setup: update MANIFEST.in
  importer: do not checksum data twice
  client: store uncompressed if compressed size is equal
  fixup! master: automatically discard feeding cells that get out-of-date
  master: automatically discard feeding cells that get out-of-date
  qa: remove useless indentation in testSafeTweak
  bench: new option to mesure ZEO perfs in matrix test
  bench: reduce number of partitions in matrix test
  storage: fix replication of creation undone

For example NEOCluster needs to shutdown gracefully, else there are
processes left for e.g. storage nodes and they dump somthing as below on
the terminal after tests completion:

    === RUN   TestLoad
    2020/10/21 14:33:00 zodb: FIXME: open ../zodb/storage/fs1/testdata/1.fs: raw cache is not ready for invalidations -> NoCache forced
    === RUN   TestLoad/py
    I: runneo.py: /tmp/neo445013868/1: Started master(s): 127.0.0.1:24661
    WARNING: This is not the recommended way to import data to NEO: you should use the Importer backend instead.
    NEO also does not implement IStorageRestoreable interface, which means that undo information is not preserved when using this tool: conflict resolution could happen when undoing an old transaction.
    Migrating from ../zodb/storage/fs1/testdata/1.fs to 127.0.0.1:24661
    Migration done in 0.19877
    --- PASS: TestLoad (0.75s)
        --- PASS: TestLoad/py (0.74s)
    PASS
    ok      lab.nexedi.com/kirr/neo/go/neo  0.749s
    (neo) (z-dev) (g.env) kirr@deco:~/src/neo/src/lab.nexedi.com/kirr/neo/go/neo$ Traceback (most recent call last):
      File "/home/kirr/src/neo/src/lab.nexedi.com/kirr/neo/neo/tests/functional/__init__.py", line 182, in start
        getattr(neo.scripts,  command).main()
      File "/home/kirr/src/neo/src/lab.nexedi.com/kirr/neo/neo/scripts/neostorage.py", line 66, in main
        app.run()
      File "/home/kirr/src/neo/src/lab.nexedi.com/kirr/neo/neo/storage/app.py", line 147, in run
        self._run()
      File "/home/kirr/src/neo/src/lab.nexedi.com/kirr/neo/neo/storage/app.py", line 178, in _run
        self.doOperation()
      File "/home/kirr/src/neo/src/lab.nexedi.com/kirr/neo/neo/storage/app.py", line 266, in doOperation
        poll()
      File "/home/kirr/src/neo/src/lab.nexedi.com/kirr/neo/neo/storage/app.py", line 87, in _poll
        self.em.poll(1)
      File "/home/kirr/src/neo/src/lab.nexedi.com/kirr/neo/neo/lib/event.py", line 155, in poll
        self._poll(blocking)
      File "/home/kirr/src/neo/src/lab.nexedi.com/kirr/neo/neo/lib/event.py", line 253, in _poll
        timeout_object.onTimeout()
      File "/home/kirr/src/neo/src/lab.nexedi.com/kirr/neo/neo/lib/event.py", line 259, in onTimeout
        on_timeout()
      File "/home/kirr/src/neo/src/lab.nexedi.com/kirr/neo/neo/storage/database/manager.py", line 207, in _deferredCommit
        self.commit()
      File "/home/kirr/src/neo/src/lab.nexedi.com/kirr/neo/neo/storage/database/manager.py", line 193, in commit
        self._commit()
      File "/home/kirr/src/neo/src/lab.nexedi.com/kirr/neo/neo/storage/database/sqlite.py", line 90, in _commit
        retry_if_locked(self.conn.commit)
      File "/home/kirr/src/neo/src/lab.nexedi.com/kirr/neo/neo/storage/database/sqlite.py", line 45, in retry_if_locked
        return f(*args)
    OperationalError: disk I/O error

This fixes

	=== RUN   TestLoad/py
	    xtesting.go:297: load 0285cbacc06d3a4c:0000000000000007: returned err unexpected:
	        have: <nil>
	        want: neo://1@127.0.0.1:27145: load 0285cbacc06d3a4c:0000000000000007: 0000000000000007: object was deleted @0285cbacc06d3a4c
	    xtesting.go:297: load 0285cbacc06d3a4c:0000000000000007: returned tid unexpected: 0285cbacc06d3a4c  ; want: 0285cbacc06d3a4c
	    xtesting.go:297: load 0285cbacc06d3a4c:0000000000000007: returned buf != nil
	    xtesting.go:297: load 0285cbad858bf2e6:0000000000000006: returned err unexpected:
	        have: <nil>
	        want: neo://1@127.0.0.1:27145: load 0285cbad858bf2e6:0000000000000006: 0000000000000006: object was deleted @0285cbad858bf2e6
	    xtesting.go:297: load 0285cbad858bf2e6:0000000000000006: returned tid unexpected: 0285cbad858bf2e6  ; want: 0285cbad858bf2e6
	    xtesting.go:297: load 0285cbad858bf2e6:0000000000000006: returned buf != nil
	    xtesting.go:315: load 7fffffffffffffff:0000000000000006: returned err unexpected:
	        have: <nil>
	        want: neo://1@127.0.0.1:27145: load 7fffffffffffffff:0000000000000006: 0000000000000006: object was deleted @0285cbad858bf2e6
	    xtesting.go:315: load 7fffffffffffffff:0000000000000006: returned tid unexpected: 0285cbad858bf2e6  ; want: 0285cbad858bf2e6
	    xtesting.go:315: load 7fffffffffffffff:0000000000000006: returned buf != nil
	    xtesting.go:315: load 7fffffffffffffff:0000000000000007: returned err unexpected:
	        have: <nil>
	        want: neo://1@127.0.0.1:27145: load 7fffffffffffffff:0000000000000007: 0000000000000007: object was deleted @0285cbacc06d3a4c
	    xtesting.go:315: load 7fffffffffffffff:0000000000000007: returned tid unexpected: 0285cbacc06d3a4c  ; want: 0285cbacc06d3a4c
	    xtesting.go:315: load 7fffffffffffffff:0000000000000007: returned buf != nil

NEO/py sends all zeros for empty data. Without this fix tests fail:

    --- FAIL: TestLoad/py (0.74s)
        xtesting.go:297: load 0285cbacc06d3a4c:0000000000000007: returned err unexpected:
            have: neo://1@127.0.0.1:25910: load 0285cbacc06d3a4c:0000000000000007: data corrupt: checksum mismatch
            want: neo://1@127.0.0.1:25910: load 0285cbacc06d3a4c:0000000000000007: 0000000000000007: object was deleted @0285cbacc06d3a4c
        xtesting.go:297: load 0285cbad858bf2e6:0000000000000006: returned err unexpected:
            have: neo://1@127.0.0.1:25910: load 0285cbad858bf2e6:0000000000000006: data corrupt: checksum mismatch
            want: neo://1@127.0.0.1:25910: load 0285cbad858bf2e6:0000000000000006: 0000000000000006: object was deleted @0285cbad858bf2e6
        xtesting.go:315: load 7fffffffffffffff:0000000000000006: returned err unexpected:
            have: neo://1@127.0.0.1:25910: load 7fffffffffffffff:0000000000000006: data corrupt: checksum mismatch
            want: neo://1@127.0.0.1:25910: load 7fffffffffffffff:0000000000000006: 0000000000000006: object was deleted @0285cbad858bf2e6
        xtesting.go:315: load 7fffffffffffffff:0000000000000007: returned err unexpected:
            have: neo://1@127.0.0.1:25910: load 7fffffffffffffff:0000000000000007: data corrupt: checksum mismatch
            want: neo://1@127.0.0.1:25910: load 7fffffffffffffff:0000000000000007: 0000000000000007: object was deleted @0285cbacc06d3a4c