Thread allocation uses two memory pools

[hkaiser]

The current thread allocation is based on a staged memory pool system. One pool is used for the thread objects (hpx::threads::thread_data) and one is used for the stack segments. We should try to unify this into one memory recycling scheme to reduce overheads.

[brycelelbach]

This has been done (albeit in a crude fashion, but it works and does not involve ripping out large chunks of the coroutines/threading code), for all schedulers except for hierarchy. I'm going to close this anyways, though; the hierarchy scheduler is very different in design from the other schedulers, and is still very experimental.