this isn't working yet, but it's a step in the right direction:
we need to call the connect callback from isc_nm_httpconnect()
asynchronously, after the connect has already returned. this
will enable dig to unlock the lookup before tcp_connected()
locks it, as it does when using tcpdns or tlsdns sockets.
this commit also removes the http_connect_t structure and stores
the connection data in a newly created http socket instead. (since
the socket structure was going to be needed to call isc__nm_connectcb()
anyway, it seemed like a good idea to make use of it to simplify
the connection code.)
4fe216e6f7