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.

lib/ns/include/ns/interfacemgr.h

@@ -65,9 +65,6 @@
 
 #define NS_INTERFACEFLAG_ANYADDR   0x01U /*%< bound to "any" address */
 #define NS_INTERFACEFLAG_LISTENING 0x02U /*%< listening */
-#define MAX_UDP_DISPATCH                           \
-	128 /*%< Maximum number of UDP dispatchers \
-	     *           to start per interface */
 /*% The nameserver interface structure */
 struct ns_interface {
 	unsigned int	   magic; /*%< Magic number. */