create and use multiple fetch dispatches

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.

bin/tests/resolver/t_resolver.c

@@ -85,7 +85,7 @@ make_resolver(dns_resolver_t **resolverp) {
 	isc_result_t result;
 
 	result = dns_resolver_create(view,
-			    task_manager, 1,
+			    task_manager, 1, 1,
 			    socket_manager,
 			    timer_manager,
 			    0, /* unsigned int options, */