See 9385706f (Fix identification issues, including a race condition
causing id conflicts).

The added test describes how the new id timestamps fix the race condition.
These timestamps could be any unique opaque values, and the protocol is
extended to exchange them along with node ids.

Internally, nodes also reuse timestamps as a marker to identify the first
NotifyNodeInformation packets from the master: since this packet is a complete
list of nodes in the cluster, any other node in the node manager has left the
cluster definitely and is removed.

The secondary masters didn't receive update about master nodes.
It's also useless to send them information about non-master nodes.

When Client (including backup master) and admin nodes are identified,
the primary master now sends them automatically all nodes with
NotifyNodeInformation, as with storage nodes.

- check address conflicts
- on invalid values, reject peer instead of dying

Listing connected/connecting nodes with a UUID is used:
- in one place by storage nodes: here, it does not matter if we skip nodes that
  aren't really identified
- in many places by the master, only for server connections, in which case we
  have equivalence with real identification

So in practice, NodeManager is only simplified to reuse the 'identified'
property of nodes.

Therefore, a client node in the node manager is always RUNNING.

Although the change applies to any node with a temporary ids (all but storage),
only clients don't have addresses and are therefore not recognizable.

After a client is disconnected from the master and before reconnecting, another
client may join the cluster and "steals" the id of the first client. This issue
leads to stuck clients, failing in loop with exceptions like the following one:

    ERROR ZODB.Connection Couldn't load state for 0x0251
    Traceback (most recent call last):
      File "ZODB/Connection.py", line 860, in setstate
        self._setstate(obj)
      File "ZODB/Connection.py", line 901, in _setstate
        p, serial = self._storage.load(obj._p_oid, '')
      File "neo/client/Storage.py", line 82, in load
        return self.app.load(oid)[:2]
      File "neo/client/app.py", line 353, in load
        data, tid, next_tid, _ = self._loadFromStorage(oid, tid, before_tid)
      File "neo/client/app.py", line 373, in _loadFromStorage
        for node, conn in self.cp.iterateForObject(oid, readable=True):
      File "neo/client/pool.py", line 91, in iterateForObject
        pt = self.app.pt
      File "neo/client/app.py", line 145, in __getattr__
        self._getMasterConnection()
      File "neo/client/app.py", line 214, in _getMasterConnection
        result = self.master_conn = self._connectToPrimaryNode()
      File "neo/client/app.py", line 246, in _connectToPrimaryNode
        handler=handler)
      File "neo/lib/threaded_app.py", line 154, in _ask
        _handlePacket(qconn, qpacket, kw, handler)
      File "neo/lib/threaded_app.py", line 135, in _handlePacket
        handler.dispatch(conn, packet, kw)
      File "neo/lib/handler.py", line 66, in dispatch
        method(conn, *args, **kw)
      File "neo/lib/handler.py", line 188, in error
        getattr(self, Errors[code])(conn, message)
      File "neo/client/handlers/__init__.py", line 23, in protocolError
        raise StorageError("protocol error: %s" % message)
    StorageError: protocol error: already connected

* master:
  coverage: CacheItem.__repr__ (client)
  New neotestrunner option for code coverage testing

* x/go:
  .
  .
  .
  X notes on partition table
  .
  .
  .
  .
  .
  .

* master:
  client: fix item eviction from cache, which could break loading from storage
  Bump protocol version for new read-only mode in BACKUPING state
  backup: Teach cluster in BACKUPING state to also serve regular ZODB clients in read-only mode
  tests/threaded: Add handy shortcuts to NEOCluster to concisely check cluster properties in tests

* master:
  client: fix item eviction from cache, which could break loading from storage
  Bump protocol version for new read-only mode in BACKUPING state
  backup: Teach cluster in BACKUPING state to also serve regular ZODB clients in read-only mode
  tests/threaded: Add handy shortcuts to NEOCluster to concisely check cluster properties in tests

`ClientCache._oid_dict` shall not have empty values. For a given oid, when the
last item is removed from the cache, the oid must be removed as well to free
memory. In some cases, this was not done.

A consequence of this bug is the following exception:

    ERROR ZODB.Connection Couldn't load state for 0x02d1e1e4
    Traceback (most recent call last):
      File "ZODB/Connection.py", line 860, in setstate
        self._setstate(obj)
      File "ZODB/Connection.py", line 901, in _setstate
        p, serial = self._storage.load(obj._p_oid, '')
      File "neo/client/Storage.py", line 82, in load
        return self.app.load(oid)[:2]
      File "neo/client/app.py", line 358, in load
        self._cache.store(oid, data, tid, next_tid)
      File "neo/client/cache.py", line 228, in store
        prev = item_list[-1]
    IndexError: list index out of range

A backup cluster for tids <= backup_tid has all data to provide regular
read-only ZODB service. Having regular ZODB access to the data can be
handy e.g. for externally verifying data for consistency between
main and backup clusters. Peeking around without disturbing main
cluster might be also useful sometimes.

In this patch:

- master & storage nodes are taught:

    * to instantiate read-only or regular client service handler depending on cluster state:
      RUNNING   -> regular
      BACKINGUP -> read-only

    * in read-only client handler:
      + to reject write-related operations
      + to provide read operations but adjust semantic as last_tid in the database
        would be = backup_tid

- new READ_ONLY_ACCESS protocol error code is introduced so that client can
  raise POSException.ReadOnlyError upon receiving it.

I have not implemented back-channel for invalidations in read-only mode (yet ?).
This way once a client connects to cluster in backup state, it won't see
new data fetched by backup cluster from upstream after client connected.

The reasons invalidations are not implemented is that for now (imho)
there is no off-hand ready infrastructure to get updates from
replicating node on transaction-by-transaction basis (it currently only
notifies when whole batch is done). For consistency verification (main
reason for this patch) we also don't need invalidations to work, as in
that task we always connect afresh to backup. So I simply only put
relevant TODOs about invalidations for now.

The patch is not very polished but should work.

/reviewed-on !4