Receive invalidation messages correctly in tablesync worker

We didn't accept any invalidation messages until the whole sync process
had finished (because it flattens all the remote transactions in the
single one).  So the sync worker didn't learn about subscription
changes/drop until it has finished.  This could lead to "orphaned" sync
workers.

Author: Petr Jelinek <petr.jelinek@2ndquadrant.com>
Reported-by: Masahiko Sawada <sawada.mshk@gmail.com>