3316.	[tuning]	Improved locking performance when recursing.
			[RT #28836]

   - Use one lock per memory pool instead of associating
     them all with a single 'pool_lock' in the dispatch manager.
   - Reduce the critical sections for qid->lock in get_dispsocket(),
     deref_portentry() and dns_dispatch_addresponse2().
   - Added a memory pool for socket events in dns_dispatch_t.
   - Add an isc_socketevent_t member in the resquery_t object, and use
     it with isc_socket_sendto2() instead of using isc_socket_sendto()
   - Tuned the memory pools in dispatch.c for better performance
     under load