Check for two_phase change at end of process_syncing_tables_for_apply.

Check for two_phase change at end of process_syncing_tables_for_apply.

Previously this function checked to see if we were ready to switch
to two_phase mode at its start, but that's silly: we should check
at the end, after we've done the work that might make us ready.
This simple change removes one sleep cycle from the time needed to
switch to two_phase mode.  In the real world that might not be
worth much, but it shaves a few seconds off the runtime for the
subscription test suite.

Nathan Bossart

Discussion: https://postgr.es/m/20221122004119.GA132961@nathanxps13