Eliminate unnecessary memory allocation in extended query protocol.

Eliminate unnecessary memory allocation in extended query protocol.

When pending messages are created, Pgpool-II did like:

(1) pmsg = pool_pending_message_create();	/* create a pending message */
(2) pool_pending_message_dest_set(pmsg, query_context)	/* set PostgreSQL node ids to be sent */
(3) pool_pending_message_query_set(pmsg, query_context);	/* add query context */
(4) pool_pending_message_add(pmsg);		/* add the pending message to the list */
(5) pool_pending_message_free_pending_message(pmsg);	/* free memory allocated by pool_pending_message_create();

The reason why pool_pending_message_free_pending_message(pmsg) is
called here is, pool_pending_message_add() creates a copy of the
pending message then add it to the list. This commit modifies
pool_pending_message_add() so that it does not create a copy of the
object and adds it to the pending messages list. This way, we can
eliminate (5) as well and it should reduce memory footprint and CPU
cycle.