pgsql: Repair two related errors in heap_lock_tuple: it was failing to

Log Message:
-----------
Repair two related errors in heap_lock_tuple: it was failing to recognize
cases where we already hold the desired lock "indirectly", either via
membership in a MultiXact or because the lock was originally taken by a
different subtransaction of the current transaction. These cases must be
accounted for to avoid needless deadlocks and/or inappropriate replacement of
an exclusive lock with a shared lock. Per report from Clarence Gardner and
subsequent investigation.

Modified Files:
--------------
pgsql/src/backend/access/heap:
heapam.c (r1.221 -> r1.222)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/heap/heapam.c.diff?r1=1.221&r2=1.222)
pgsql/src/backend/access/transam:
multixact.c (r1.21 -> r1.22)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/multixact.c.diff?r1=1.21&r2=1.22)
pgsql/src/backend/utils/time:
tqual.c (r1.99 -> r1.100)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/time/tqual.c.diff?r1=1.99&r2=1.100)
pgsql/src/include/access:
multixact.h (r1.10 -> r1.11)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/access/multixact.h.diff?r1=1.10&r2=1.11)