Avoid transaction-commit race condition while receiving a NOTIFY message.

Avoid transaction-commit race condition while receiving a NOTIFY message.

Use TransactionIdIsInProgress, then TransactionIdDidCommit, to distinguish
whether a NOTIFY message's originating transaction is in progress,
committed, or aborted.  The previous coding could accept a message from a
transaction that was still in-progress according to the PGPROC array;
if the client were fast enough at starting a new transaction, it might fail
to see table rows added/updated by the message-sending transaction.  Which
of course would usually be the point of receiving the message.  We noted
this type of race condition long ago in tqual.c, but async.c overlooked it.

The race condition probably cannot occur unless there are multiple NOTIFY
senders in action, since an individual backend doesn't send NOTIFY signals
until well after it's done committing.  But if two senders commit in close
succession, it's certainly possible that we could see the second sender's
message within the race condition window while responding to the signal
from the first one.

Per bug #9557 from Marko Tiikkaja.  This patch is slightly more invasive
than what he proposed, since it removes the now-redundant
TransactionIdDidAbort call.

Back-patch to 9.0, where the current NOTIFY implementation was introduced.