Fix query-lifespan memory leakage in repeatedly executed hash joins.

ExecHashTableCreate allocated some memory that wasn't freed by
ExecHashTableDestroy, specifically the per-hash-key function information.
That's not a huge amount of data, but if one runs a query that repeats
a hash join enough times, it builds up.  Fix by arranging for the data
in question to be kept in the hashtable's hashCxt instead of leaving it
"loose" in the query-lifespan executor context.  (This ensures that we'll
also clean up anything that the hash functions allocate in fn_mcxt.)

Per report from Amit Khandekar.  It's been like this forever, so back-patch
to all supported branches.

Discussion: https://postgr.es/m/CAJ3gD9cFofAWGvcxLOxDHC=B0hjtW8yGmUsF2hdGh97CM38=7g@mail.gmail.com

src/backend/executor/nodeHash.c

@@ -472,7 +472,8 @@ ExecHashTableCreate(HashState *state, List *hashOperators, bool keepNulls)
 	 * Initialize the hash table control block.
 	 *
 	 * The hashtable control block is just palloc'd from the executor's
-	 * per-query memory context.
+	 * per-query memory context.  Everything else should be kept inside the
+	 * subsidiary hashCxt or batchCxt.
 	 */
 	hashtable = (HashJoinTable) palloc(sizeof(HashJoinTableData));
 	hashtable->nbuckets = nbuckets;
@@ -514,6 +515,22 @@ ExecHashTableCreate(HashState *state, List *hashOperators, bool keepNulls)
 		   hashtable, nbatch, nbuckets);
 #endif
 
+	/*
+	 * Create temporary memory contexts in which to keep the hashtable working
+	 * storage.  See notes in executor/hashjoin.h.
+	 */
+	hashtable->hashCxt = AllocSetContextCreate(CurrentMemoryContext,
+											   "HashTableContext",
+											   ALLOCSET_DEFAULT_SIZES);
+
+	hashtable->batchCxt = AllocSetContextCreate(hashtable->hashCxt,
+												"HashBatchContext",
+												ALLOCSET_DEFAULT_SIZES);
+
+	/* Allocate data that will live for the life of the hashjoin */
+
+	oldcxt = MemoryContextSwitchTo(hashtable->hashCxt);
+
 	/*
 	 * Get info about the hash functions to be used for each hash key. Also
 	 * remember whether the join operators are strict.
@@ -540,22 +557,6 @@ ExecHashTableCreate(HashState *state, List *hashOperators, bool keepNulls)
 		i++;
 	}
 
-	/*
-	 * Create temporary memory contexts in which to keep the hashtable working
-	 * storage.  See notes in executor/hashjoin.h.
-	 */
-	hashtable->hashCxt = AllocSetContextCreate(CurrentMemoryContext,
-											   "HashTableContext",
-											   ALLOCSET_DEFAULT_SIZES);
-
-	hashtable->batchCxt = AllocSetContextCreate(hashtable->hashCxt,
-												"HashBatchContext",
-												ALLOCSET_DEFAULT_SIZES);
-
-	/* Allocate data that will live for the life of the hashjoin */
-
-	oldcxt = MemoryContextSwitchTo(hashtable->hashCxt);
-
 	if (nbatch > 1 && hashtable->parallel_state == NULL)
 	{
 		/*