• move some of the scheduler code around and rename some types
  • core::scheduler::scheduler::Scheduler becomes core::scheduler::thread_per_host::ThreadPerHostSched
  • core::scheduler::thread_per_host::ThreadPerHostSched is re-exported as just core::scheduler::ThreadPerHostSched
  • core::scheduler::workpool::WorkPool becomes core::scheduler::pools::bounded::ParallelismBoundedThreadPool
• add a scheduler wrapper so that we can support other schedulers
  • core::scheduler::Scheduler
• add an experimental scheduler configuration option

I'm not happy with the names, but we need something for now. The future thread-per-core scheduler/pool is tentatively named ThreadPerCoreSched and pools::unbounded::UnboundedThreadPool.