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/include/nodes/memnodes.h

@@ -60,6 +60,9 @@ typedef struct MemoryContextData
 	MemoryContext nextchild;	/* next child of same parent */
 	char	   *name;			/* context name (just for debugging) */
 	bool		isReset;		/* T = no space alloced since last reset */
+#ifdef USE_ASSERT_CHECKING
+	bool		allowInCritSection;	/* allow palloc in critical section */
+#endif
 } MemoryContextData;
 
 /* utils/palloc.h contains typedef struct MemoryContextData *MemoryContext */