BUG/MEDIUM: Special-case http_proxy when dealing with outgoing connections.

BUG/MEDIUM: Special-case http_proxy when dealing with outgoing connections.

http_proxy is special, because it creates its connection and conn_stream
earlier. So in assign_server(), check that the connection associated with
the conn_stream has a destination address set, and in connect_server(),
use the connection and the conn_stream already attached to the
stream_interface, instead of looking for a connection in the session, and
creating a new conn_stream.