Description of problem:
[RFE] Async Refactoring

1. Use asio’s stackful coroutines to clean up some of the larger sync coroutines, and unify them with existing code that understands optional_yield

2. Hook up asio’s completions to issue completions within RGWCoroutinesManager so we can gradually replace pieces of the sync coroutine hierarchy

3. Replace the RGWAsyncRadosProcessor thread pool (which is used to call synchronous code within sync) with use of optional_yield