mk_scheduler: Remove mk_sched_update_conn_status().

Unsused function.
mk_scheduler: Remove prototype mk_sched_update_thread_status().

No associated function.
en/mconf: Add example usage to Listen comment.
mk_scheduler: Register connections with hooks.

Change prototypes and logic to support atomic registration of client
connections with a connection hook set.

mk_sched_add_client() and mk_sched_register_client() replaced with
mk_sched_register().
mk_sched_remove_client() replaced with mk_sched_unregister().

mk_sched_connection_init() added, initialize a connection that is
registered with scheduler, but can't be found with
mk_sched_connection_get().
mk_epoll: Remove hooks argument from mk_epoll_init().

Hooks argument was workaround which isn't needed anymore.
mk_socket: mk_sched_get_connection() renamed mk_sched_connection_get().
mk_server: Use hook set from Listen configuration.

New client connections are assigned different protocol hooks set
depending on which Listen socket they arrived on.
mk_request: mk_sched_get_connection() renamed mk_sched_connection_get().
mk_plugin_next: Scheduler prototypes changed.
mk_plugin: Scheduler prototypes changed.
mk_http: Compensate for mk_sched_unregister() not closing fd.
mk_scheduler: Re-add timeout support.

Add mk_sched_connection_timeout() for setting an arbitrary timeout.
mk_request: Connection member arrival_time removed.
mk_http: Refresh timeout on keep-alive connections.

Refresh timout to config->timeout seconds when all requests are served
on keep-alive requests.

Fix multiple tcp_cork on/off.
mk_queue: Fix resource leakage in mk_queue_mark_used().
mk_scheduler: Fix unused variable warning.