pgsql: Fix oversight in async-commit patch: there were some places in

Log Message:
-----------
Fix oversight in async-commit patch: there were some places in heapam.c
that still thought they could set HEAP_XMAX_COMMITTED immediately after
seeing the other transaction commit. Make them use the same logic as
tqual.c does to determine if the hint bit can be set yet.

Modified Files:
--------------
pgsql/src/backend/access/heap:
heapam.c (r1.236 -> r1.237)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/heap/heapam.c?r1=1.236&r2=1.237)
pgsql/src/backend/utils/time:
tqual.c (r1.103 -> r1.104)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/time/tqual.c?r1=1.103&r2=1.104)
pgsql/src/include/utils:
tqual.h (r1.67 -> r1.68)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/utils/tqual.h?r1=1.67&r2=1.68)