Reduce cost of LocalPop when local queue empty

Reduce cost of LocalPop when local queue empty

- Change LocalPop to be parameterless and return the IThreadPoolWorkItem rather than returning a bool and having the IThreadPoolWorkItem in an out arg.
- Refactor LocalPop slightly to have an inlineable upfront check for whether the local queue is empty, and only call the non-inlineable method if there's something that could potentially be popped