[runtime] No way to remove coroutines from timer

[iyzhang]

Description

While we have moved the Timer over to Yielders, we now have no way to check if the yielder is still valid when the timer goes off.

How to Reproduce

1. Create an active socket and connect.
2. Finish connecting and then immediately close the socket.
3. Move time forward by a lot.
4. The timer will go off but the socket will already have been closed and all of its coroutines cleaned up, causing a panic in the scheduler as it tries to run the background coroutine.

Expected Behavior

When sockets are destroyed (e.g., when moving from active open to established or established to closed), we should clean up all coroutines and data structures that might have references to the socket.

Related Issues

#597