pgsql: Ensure that top level aborts call XLogSetAsyncCommit().

Log Message:
-----------
Ensure that top level aborts call XLogSetAsyncCommit(). Not doing
so simply leads to data waiting in wal_buffers which then causes
later commits to potentially do emergency writes and for all forms
of replication to be potentially delayed without need or benefit.
Issue pointed out exactly by Fujii Masao, following bug report
by Robert Haas on a separate though related topic.

Modified Files:
--------------
    pgsql/src/backend/access/transam:
        xact.c (r1.290 -> r1.291)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/xact.c?r1=1.290&r2=1.291)