Fix walsender handling of postmaster shutdown, to not go into endless loop.

Fix walsender handling of postmaster shutdown, to not go into endless loop.

This bug was introduced by my patch to use the regular die/quickdie signal
handlers in walsender processes. I tried to make walsender exit at next
CHECK_FOR_INTERRUPTS() by setting ProcDiePending, but that's not enough, you
need to set InterruptPending too. On second thoght, it was not a very good
way to make walsender exit anyway, so use proc_exit(0) instead.

Also, send a CommandComplete message before exiting; that's what we did
before, and you get a nicer error message in the standby that way.

Reported by Thom Brown.