Add support for external thread events

Add support for external thread events

Add support for external thread events.

See asio_external_thread_event.h for details about usage.

This is the first version that compiled, the code was not executed even
once, but wanted to get it out early to unblock @hannesr. Thus the [RFC]
flag.

My next step is to implement asio-compatible equivalent of
fb_call_user_func_async() that will make it possible to easily test the
new functonality by simulating async stuff with sleep(), etc. on the PHP
side.

There are 2 major mostly lockless synchronization points that needs
careful review:

- queue of ready external thread events (asio_session.{cpp.h})
- shutdown cleanup (asio_external_thread_event.{cpp,h})