http-client: Changed struct http_client_host_port into a struct http_client_queue...

http-client: Changed struct http_client_host_port into a struct http_client_queue object.
Peer and request objects now reference the queue object directly rather
than the host object. This way, there is no need to find the matching
host:port in the host anymore. This makes the queueing structure more
intuitive and more efficient. This is a first step towards support for
connecting to HTTP services through unix sockets or directing requests at
specific hosts (so not from the URL). This patch also fixes a potential
timeout leak (to_connect) in http_client_host_port (now http_client_queue)
and makes sure it is moved during switch_ioloop(). Finally it updates the
structure comment at the top of http-client.c.