Expose something like gevent.threadpool.ThreadResult?

[jamadden]

There may be a need, based on #1739.

The changes to AsyncResult that fix the hang in #1739 actually come close, but it's not a thread-safe object either. I don't desire to make it fully thread-safe unless we can guarantee extremely low-overhead for its intended use case.

[jamadden]

Another useful primitive might be a thread-safe way to schedule a callback in another thread's hub. (I feel like that's come up before…) That's very primitive, though, given the limitations on callbacks in this context (no blocking API, musn't raise exceptions, etc), so it's possibly only useful for gevent internals.