Implement seamless TCP connection reuse in dns_dispatch

Implement seamless TCP connection reuse in dns_dispatch

Previously, the user of dns_dispatch API had to first call
dns_dispatch_gettcp() and if that failed create a new TCP dispatch with
dns_dispatch_createtcp().  This has been changed and the TCP connection
reuse happens transparently inside dns_dispatch_createtcp().  There are
separate buckets for dns_resolver, dns_request and dns_xfrin units, so
these don't get mixed together.