Facilitate graceful removal of timers/handles in worker process

[kriskowal]

As written, our _worker thunk will retain a child process with a timer for health checks among other things that keep the event loop running even if instructed to shut down. We should in the future, send a message to the worker via the IPC channel that shuts these handles down.

[kriskowal]

As a corollary, the worker should self report when connections are finished to the supervisor. The supervisor should track how many outstanding open connections each worker is keeping. This could eventually be an input for load balancing decisions.