This is required (or maybe strongly implied) by the IStorage interface.

Also, rearrange the order of operations in _process_invalidations.

Each of these changes avoid a potential race when a connection starts
a transaction while invalidateTransaction is being called. Basically,
we don't want a connection to get a lastTransaction corresponding to
invalidations it hasn't processed.

This neither fixes nor provokes any test failures. :) The need for
this change was discovered via code inspection. It's hard to see how
to test the race without getting being insanely whitebox.