Add threadpool_scheduler support for Android/Apple/Unix

[bghgary]

There is a stop gap solution for other platforms that doesn't actually use a threadpool for the threadpool_scheduler. This should be implemented for Android/Apple using platform specific APIs.

[unicomp21]

@bghgary does arcana work on Linux?

[bghgary]

Yes, are you seeing an issue?

[unicomp21]

No, I just wan't sure, thanks!