Make the number of CLOG buffers adaptive, based on shared_buffers.

Previously, this was hardcoded: we always had 8.  Performance testing
shows that isn't enough, especially on big SMP systems, so we allow it
to scale up as high as 32 when there's adequate memory.  On the flip
side, when shared_buffers is very small, drop the number of CLOG buffers
down to as little as 4, so that we can start the postmaster even
when very little shared memory is available.

Per extensive discussion with Simon Riggs, Tom Lane, and others on
pgsql-hackers.

src/backend/storage/lmgr/lwlock.c

@@ -171,7 +171,7 @@ NumLWLocks(void)
 	numLocks += MaxBackends + NUM_AUXILIARY_PROCS;
 
 	/* clog.c needs one per CLOG buffer */
-	numLocks += NUM_CLOG_BUFFERS;
+	numLocks += CLOGShmemBuffers();
 
 	/* subtrans.c needs one per SubTrans buffer */
 	numLocks += NUM_SUBTRANS_BUFFERS;