pgsql: Divide the lock manager's shared state into 'partitions', so as

Log Message:
-----------
Divide the lock manager's shared state into 'partitions', so as to
reduce contention for the former single LockMgrLock.  Per my recent
proposal.  I set it up for 16 partitions, but on a pgbench test this
gives only a marginal further improvement over 4 partitions --- we need
to test more scenarios to choose the number of partitions.

Modified Files:
--------------
    pgsql/src/backend/access/transam:
        twophase.c (r1.17 -> r1.18)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/twophase.c.diff?r1=1.17&r2=1.18)
    pgsql/src/backend/storage/ipc:
        procarray.c (r1.8 -> r1.9)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/ipc/procarray.c.diff?r1=1.8&r2=1.9)
    pgsql/src/backend/storage/lmgr:
        README (r1.18 -> r1.19)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/lmgr/README.diff?r1=1.18&r2=1.19)
        deadlock.c (r1.37 -> r1.38)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/lmgr/deadlock.c.diff?r1=1.37&r2=1.38)
        lock.c (r1.161 -> r1.162)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/lmgr/lock.c.diff?r1=1.161&r2=1.162)
        lwlock.c (r1.35 -> r1.36)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/lmgr/lwlock.c.diff?r1=1.35&r2=1.36)
        proc.c (r1.169 -> r1.170)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/lmgr/proc.c.diff?r1=1.169&r2=1.170)
    pgsql/src/include/storage:
        lock.h (r1.92 -> r1.93)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/storage/lock.h.diff?r1=1.92&r2=1.93)
        lwlock.h (r1.23 -> r1.24)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/storage/lwlock.h.diff?r1=1.23&r2=1.24)
        proc.h (r1.84 -> r1.85)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/storage/proc.h.diff?r1=1.84&r2=1.85)