eventloop: various registration management fixes

[jkarneges]

This fixes a few things:

• Reactor registrations are set up to use a persistent waker. Without this, the reactor triggers the waker only once and then discards it.
• Registration readiness is cleared after processing. Without this, the reactor won't trigger the registration's waker again after the first event.
• Timer registrations are automatically removed after activation.

[deg4uss3r]

Just 1 nit.

[deg4uss3r]

This can be simplified down since we're just throwing away the error:

Suggested change

	match self.regs.remove(id) {
	Ok(()) => Ok(()),
	Err(_) => Err(EventLoopError),
	}
	self.regs.remove(id).map_err(|_| EventLoopError)