Experimental support for more scalable coalesce

I'm opening this pull request to get your feedback on this feature--it's not ready for merging, and i'm trying to get it there. There are 2 problems I'm having now:

1. Even though I'm using `advance!`, the time doesn't seem to be advancing for the callback that's scheduled by the `every!`. Could this be due to the `with-redefs` in `time.controlled`, or should I be reviewing my implementation? The only thing that I haven't tested is using `call-rescue` on the children rather than invoking them directly (as I have been using previously).
1. I know that the `every!` task should be canceled when the callback is empty. Do you mind (stylewise) if I use the `ConcurrentHashMap` as the monitor for guarding the creation/deletion of the task with `lock`?