expand SessionEvent API to allow ::clear_events() to work correctly.

expand SessionEvent API to allow ::clear_events() to work correctly.

clear_events() must run in realtime context, which is likely to be asynchronous
with respect to the thread that calls it. So allow caller to pass in a functor
that will be executed (also in realtime context) after the clear is done.

Additionally, allow for a cross-thread callback to the event loop/thread which
initiated/allocated the clear event request so that it can flush its own pending
loop. This part probably isn't necessary but doesn't hurt and is a useful model.
The event would be placed back in the free list at the next event allocation
by the calling thread anyway.