Use ΔFtail.Track on every READ, and query accumulated ΔFtail upon
receiving ZODB invalidation to query it about which blocks of which
files have been changed. Then invalidate those blocks in OS file cache.

See added documentation to wcfs.go and notes.txt for details.

Now the filesystem is no longer stale: it provides view of data
that is uptodate wrt changes on ZODB storage.

Some preliminary history:

kirr/wendelin.core@9b4a42a3    X invalidation design draftly settled
kirr/wendelin.core@27d91d47    X δFtail settled
kirr/wendelin.core@33e0dfce    X ΔTail draftly done
kirr/wendelin.core@822366a7    X keeping fd to root opened prevents the filesystem from being unmounted
kirr/wendelin.core@89ad3a79    X Don't keep ZBigFile activated during whole current transaction
kirr/wendelin.core@245511ac    X Give pointer on from where to get nxd-fuse.ko
kirr/wendelin.core@d1cd128c    X Hit FUSE-related deadlock
kirr/wendelin.core@d134ee44    X FUSE lookup deadlock should be hopefully fixed
kirr/wendelin.core@0e60e9ff    X wcfs: Don't noise ZWatcher trace logs with "select ..."
kirr/wendelin.core@bf9a7405    X No longer rely on ZODB cache invariant for invalidations