Make dns_dispatch bound to threads

Make dns_dispatch bound to threads

Instead of high number of dispatches (4 * named_g_udpdisp)[1], make the
dispatches bound to threads and make dns_dispatchset_t create a dispatch
for each thread (event loop).

This required couple of other changes:

1. The dns_dispatch_createudp() must be called on loop, so the isc_tid()
   is already initialized - changes to nsupdate and mdig were required.

2. The dns_requestmgr had only a single dispatch per v4 and v6.  Instead
   of using single dispatch, use dns_dispatchset_t for each protocol -
   this is same as dns_resolver.