Refactor dispatch, resolver and request

Since every dispsock was associated with a dispentry anyway (though not
always vice versa), the members of dispsock have been combined into
dispentry, which is now reference-counted.  dispentry objects are now
attached before connecting and detached afterward to prevent races
between the connect callback and dns_dispatch_removeresponse().

Dispatch and dispatchmgr objects are now reference counted as well, and
the shutdown process has been simplified.  reference counting of
resquery and request objects has also been cleaned up significantly.

dns_dispatch_cancel() now flags a dispentry as having been canceled, so
that if the connect callback runs after cancellation, it will not
initiate a read.

The isblackholed() function has been simplified.

bin/tools/mdig.c

@@ -2175,7 +2175,7 @@ main(int argc, char *argv[]) {
 	dns_requestmgr_detach(&requestmgr);
 
 	dns_dispatch_detach(&dispatchvx);
-	dns_dispatchmgr_destroy(&dispatchmgr);
+	dns_dispatchmgr_detach(&dispatchmgr);
 
 	isc_task_shutdown(task);
 	isc_task_detach(&task);