Fix memory leak during long distributed insert

Tuple expression context memory context is not properly
reset during chunk dispatch execution which eventually
consumes all available memory during the query execution:

INSERT INTO test_table
  SELECT now() - random() * interval '2 years', (i/100)::text, random()
FROM
  generate_series(1,700000) AS sub(i);

This problem does not reproduces for a distributed hypertables
with disabled batching and for a regular hypertables.
Because luckly the tuple expression context got freed during
the ModifyTable node execution.

src/chunk_dispatch_state.c

@@ -116,6 +116,9 @@ chunk_dispatch_exec(CustomScanState *node)
 	if (TupIsNull(slot))
 		return NULL;
 
+	/* Reset the per-tuple exprcontext */
+	ResetPerTupleExprContext(estate);
+
 	/* Switch to the executor's per-tuple memory context */
 	old = MemoryContextSwitchTo(GetPerTupleMemoryContext(estate));