Temporarily move dns_tcpmsg functionality into dispatch

Temporarily move dns_tcpmsg functionality into dispatch

Continuing the effort to move all uses of the isc_socket API into
dispatch.c, this commit removes the dns_tcpmsg module entirely, as
dispatch was its only caller, and moves the parts of its functionality
that were being used into the dispatch module.

This code will be removed when we switch to using netmgr TCPDNS.