Working on reactor.

- Implement event subscription.
- Refactor the reactor/handler relation so we don't pass
  reactors all over the place.
- Update the handler life-cycle for reactor-owned
  handlers
- Remove on_open, on_close events (use ctors, dtors).
- Removed reactor dependency in hanlder registry and
  timer queue.
Fixing build errors form reactor changes.
Initial implementation of acceptor.
Initial implementation of the connector pattern.