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.