Move GlobalThreadPoolList.h into folly/concurrency/
Summary:
[Folly] Move `GlobalThreadPoolList.h` into `folly/concurrency/`.
This seems like a better place than the top-level. It seems reasonable because this has *something* to do with concurrency. This is used by executors, but is not itself an executor, so it was not clear that it should go to `folly/executors/`, although that also seems like quite the reasonable choice.
Reviewed By: Orvid
Differential Revision:
D5985142
fbshipit-source-id:
6117d133757af74023d6cffb838741da83d173bd