Ensure the loop callback is scheduled when the ready queue is not empty

Ensure the loop callback is scheduled when the ready queue is not empty

Summary: Previously we'd call ensureLoopScheduled() but it'd be a no-op since the loop was already scheduled. Delaying the call to ensureLoopScheduled() fixes the issue

Test Plan: unit tests (FiberManager.yieldTest fails without the changes to FiberManager-inl.h)

Reviewed By: andrii@fb.com

Subscribers: folly-diffs@, yfeldblum, chalfant

FB internal diff: D1993686

Signature: t1:1993686:1429070253:af933abbbbb33868a402f1d643e4e6f5fef1be83