Fix and enhance the assertion of no palloc's in a critical section.

The assertion failed if WAL_DEBUG or LWLOCK_STATS was enabled; fix that by
using separate memory contexts for the allocations made within those code
blocks.

This patch introduces a mechanism for marking any memory context as allowed
in a critical section. Previously ErrorContext was exempt as a special case.

Instead of a blanket exception of the checkpointer process, only exempt the
memory context used for the pending ops hash table.

src/backend/storage/lmgr/proc.c

@@ -411,8 +411,9 @@ InitProcess(void)
 
 	/*
 	 * Now that we have a PGPROC, we could try to acquire locks, so initialize
-	 * the deadlock checker.
+	 * local state needed for LWLocks, and the deadlock checker.
 	 */
+	InitLWLockAccess();
 	InitDeadLockChecking();
 }