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/byname_test.c

@@ -303,7 +303,7 @@ main(int argc, char *argv[]) {
 			INSIST(disp6 != NULL);
 		}
 
-		RUNTIME_CHECK(dns_view_createresolver(view, taskmgr, 10,
+		RUNTIME_CHECK(dns_view_createresolver(view, taskmgr, 10, 1,
 						      socketmgr,
 						      timermgr, 0,
 						      dispatchmgr,