mark MVCCAdapter instance's tpc_finish callbacks

[d-maurer]

Since ZODB>=5.6 lastTransaction must change after invalidation processing. The problem reported in "zopefoundation/ZEO#166" demonstrates that this is necessary for the invalidations from a local tpc_finish as well.

MVCCAdapter instances put those invalidations into the tpc_finish callback. This implies that lastTransaction must change after those callbacks are run. The test ZODB.tests.BasicStorage.BasicStorage.check_tid_ordering_w_commit uses the tpc_finish callback to sample the result of lastTransaction. It requires that lastTransaction changes before the sampling. Obviously, both requirements contradict one another.

That the test currently succeeds for all storages almost surely means that those storages do not yet fulfill the requirement introduced by ZODB==5.6. The optimal approach likely would be to modify all storages and adapt the test accordingly. Because I do not want to change all storages at the moment, I opted for a different (slightly hacky) solution:
MVCCAdapter instances set callback.invalidateTransaction = True for their tpc_finish callbacks. This allows a storage implementation to distinguish callbacks from MVCCAdapter instances (lastTransaction must change after the callback) from other callbacks (e.g. from tests where lastTransaction might be expected to change before the callback).

[d-maurer]

The explanation provided by @navytux in "#307 (comment)" shows that this is not necessary.