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    FileStorageError:  The database has already been packed to a
    later time or no changes have been made since the last pack

exception.  Instead that message is logged (at INFO level), and
the pack attempt simply returns then (no pack is performed).

Incidentally, this should repair frequent reports of failure in the
new checkPackLotsWhileWriting test.  On non-Windows systems, it
seems that the worker thread often didn't get enough cycles to commit
a change between the main thread's attempts to run pack() (and so
the exception above got raised then).

failures in the ZEO FileStorage pack-while-writing tests, where they die
with
    CorruptedError: ... transaction with checkpoint flag set

By coincidence, the same kind of death during pack() was reported on
zodb-dev today(!).  The new checkPackLotsWhileWriting test provokes this
on my box reliably (failed each time it was run).

The problem appears to be that pack's idea of where a FileStorage
ends was obtained merely by seeking to the end of the file.  But if
a transaction is in progress, there can be an extra "not really there
yet" transaction at the end of the file, and when either buildPackIndex()
or findReachableFromFuture() bumped into a such a thing, the CorruptedError
above got raised.  This is always rare, and was exceedingly rare before
because only one pack per test was tried.  The new test tries packing
repeatedly while a thread keeps hammering the database, so is much more
likely to provoke the problem.

The fix amounts to passing FileStorage._pos to the pack code, telling
the latter directly where the legit transactions in the file end.