Philip Withnall authored Dec 16, 2013 and Olivier Crête committed Jan 31, 2014

If the Component’s I/O receiver machinery is invoked from a thread which
can’t acquire the main context specified for the I/O callbacks, the
callbacks need to be queued as idle handlers in that main context.

This is needed for the case where blocking reads are being performed in
one thread, with their callbacks needing to be delivered in another
thread.

This introduces a new fine-grained lock to Component: io_mutex. This
protects accesses to Component->io_callback, Component->io_user_data and
Component->pending_io_callbacks. If being locked at the same time as the
main agent lock, it must always be locked afterwards, but the agent lock
does not *have* to be held in order to lock io_mutex.