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

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.