Summary:
The WorkersProvider can be used to collect OS thread IDs of all active
threads consuming tasks from the CPUThreadPoolExecutor queue. The
WorkersProvider::KeepAlive holds a read lock for the `osThreadIds_` list here to
block the active threads from exiting (they will need to acquire a wlock on the same list on exit)
while the observer captures the backtraces.
The actual logic to captures the traces in coming in a subsequent diff.

Reviewed By: mshneer

Differential Revision: D30133823

fbshipit-source-id: 5bcdc52c55077b53fe8ec3be6d2413cac4436125