Cancel statements when closing connection at transaction end.

That's important because the currently running statement on a worker
might continue to hold locks and consume resources, even after the
connection is closed.  Unfortunately postgres will only notice closed
connections when reading from / writing to the network. That might
only happen much later.

src/backend/distributed/connection/connection_management.c

@@ -718,8 +718,7 @@ AfterXactHostConnectionHandling(ConnectionHashEntry *entry, bool isCommit)
 			PQstatus(connection->pgConn) != CONNECTION_OK ||
 			PQtransactionStatus(connection->pgConn) != PQTRANS_IDLE)
 		{
-			PQfinish(connection->pgConn);
+			ShutdownConnection(connection);
-			connection->pgConn = NULL;
 
 			/* unlink from list */
 			dlist_delete(iter.cur);