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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

Added API to create a set of UDP dispatches which can be shared
round-robin style when making upstream queries for authoritative
data; this should reduce lock contention in the query source
dispatch.

                        and stub_callback were using it. [RT #26732]

                        [RT #27631]