mk_connection: Add struct mk_conn_hooks.

Implement per-connection event handling hooks.
mk_http: Implement HTTP mk_conn_hooks.
mk_scheduler: Set mk_conn_hooks for connections.

Add mk_conn_hooks argument to mk_sched_register_client() and call
mk_conn_hook_add() and mk_conn_hook_del() where needed.
monkey: Call mk_conn_init() and mk_http_hooks_init().

Initialize mk_connection state and HTTP mk_conn_hooks.
mk_epoll: Call connection hooks in epoll loop.

Replace epoll handlers with per connection hooks.
mk_plugin: epoll_init api function change.
mk_plugin: Fix mk_plugin_http_request_end().
mk_plugin: Add PLUGIN_EVENT hooks.