Merge branch 'master' into t2

* master: (40 commits)
  fixup! wcfs: Handle ZODB invalidations
  wcfs/internal/mm: Complete the package
  fixup! wcfs: client: Provide client package to care about isolation protocol details
  lib/zodb: zconn_at: Fix how ZODB4 is asserted to be patched
  lib/zodb: zstor_2zurl: Explicitly reject MappingStorage
  bigfile/zodb: Teach ZBigFile backend to use WCFS
  wcfs: client: Provide virtmem integration
  wcfs: client: Add wczsync package to maintain WCFS connection in sync to ZODB connection
  lib/zodb: Teach zconn_at to work on ZODB4
  lib/zodb: Add ZODB.Connection.onShutdownCallback
  lib/zodb: Teach Connection.onResyncCallback to work on ZODB4
  bigfile/py: Allow PyBigFile backend to expose "mmap overlay" functionality
  bigfile/virtmem: Introduce "mmap overlay" mode
  wcfs: client: Provide client package to care about isolation protocol details
  wcfs: Provide isolation to clients
  wcfs: Handle ZODB invalidations
  wcfs: Add FileSock FUSE utility
  wcfs: zdata: ΔFtail
  wcfs: xbtree: ΔBtail
  wcfs: xbtree: BTree-diff algorithm
  ...