When there is a conflict (on any object, but on ZBlk in particular) ZODB
machinery calls its ._p_invalidate() twice:

  File ".../wendelin.core/bigfile/tests/test_filezodb.py", line 661, in test_bigfile_filezodb_vs_conflicts
    tm2.commit()    # this should raise ConflictError and stay at 11 state
  File ".../transaction/_manager.py", line 111, in commit
    return self.get().commit()
  File ".../transaction/_transaction.py", line 271, in commit
    self._commitResources()
  File ".../transaction/_transaction.py", line 414, in _commitResources
    self._cleanup(L)
  File ".../transaction/_transaction.py", line 426, in _cleanup
    rm.abort(self)
  File ".../ZODB/Connection.py", line 436, in abort
    self._abort()
  File ".../ZODB/Connection.py", line 479, in _abort
    self._cache.invalidate(oid)
  File ".../wendelin.core/bigfile/file_zodb.py", line 148, in _p_invalidate
    traceback.print_stack()

and

  File ".../wendelin.core/bigfile/tests/test_filezodb.py", line 661, in test_bigfile_filezodb_vs_conflicts
    tm2.commit()    # this should raise ConflictError and stay at 11 state
  File ".../transaction/_manager.py", line 111, in commit
    return self.get().commit()
  File ".../transaction/_transaction.py", line 271, in commit
    self._commitResources()
  File ".../transaction/_transaction.py", line 416, in _commitResources
    self._synchronizers.map(lambda s: s.afterCompletion(self))
  File ".../transaction/weakset.py", line 59, in map
    f(elt)
  File ".../transaction/_transaction.py", line 416, in <lambda>
    self._synchronizers.map(lambda s: s.afterCompletion(self))
  File ".../ZODB/Connection.py", line 831, in _storage_sync
    self._flush_invalidations()
  File ".../ZODB/Connection.py", line 539, in _flush_invalidations
    self._cache.invalidate(invalidated)
  File ".../wendelin.core/bigfile/file_zodb.py", line 148, in _p_invalidate
    traceback.print_stack()

i.e. first invalidation is done by commit cleanup:

    https://github.com/zopefoundation/transaction/blob/1.4.4/transaction/_transaction.py#L414
    https://github.com/zopefoundation/ZODB/blob/3.10/src/ZODB/Connection.py#L479

and then Connection.afterCompletion() flushes invalidation again:

    https://github.com/zopefoundation/transaction/blob/1.4.4/transaction/_transaction.py#L416
    https://github.com/zopefoundation/ZODB/blob/3.10/src/ZODB/Connection.py#L833
    https://github.com/zopefoundation/ZODB/blob/3.10/src/ZODB/Connection.py#L539

If there was no conflict - there will be no ConflictError raised and
thus no Transaction._cleanup() done in its ._commitResources() ->
invalidation called only once. But with ConflictError - it is twice.

Adjust ZBlk._p_invalidate() not to delve into real invalidation more
than once - else we will fail, as ZBlk._v_zfile becomes unbound after
invalidation done the first time.