pgsql: Use the standard lock manager to establish priority order when

Log Message:
-----------
Use the standard lock manager to establish priority order when there
is contention for a tuple-level lock.  This solves the problem of a
would-be exclusive locker being starved out by an indefinite succession
of share-lockers.  Per recent discussion with Alvaro.

Modified Files:
--------------
    pgsql/src/backend/access/heap:
        heapam.c (r1.188 -> r1.189)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/heap/heapam.c.diff?r1=1.188&r2=1.189)
    pgsql/src/backend/storage/lmgr:
        lmgr.c (r1.72 -> r1.73)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/lmgr/lmgr.c.diff?r1=1.72&r2=1.73)
    pgsql/src/include/storage:
        lmgr.h (r1.47 -> r1.48)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/storage/lmgr.h.diff?r1=1.47&r2=1.48)