Will the pool correctly end if unfillable requests error out?

[myndzi]

Since request timeouts were added, this case may not have been considered. Investigate and/or fix the case where .end() is called while there are queued requests waiting that time out with an error.

[myndzi]

This will indeed loop infinitely, but doesn't apply just to timeouts; if, for example, the underlying resource goes unavailable, a new request is made, and the pool is ended without the source ever becoming available again, the requests will keep retrying until they hit their timeout. This behavior makes sense, but due to the lazy cleanup of timed out requests, an extra step needs to be taken to ensure 'drain' is called and shutdown is completed when the only things left are timed out requests. That's being taken care of, but it's a bit of an ugly hack.

Strongly starting to feel like a fairly significant rewrite is in order, probably promise-based, since I was unable to keep this code as clean as I'd like with callbacks.