Fix bug in circular singleton creation detection

Fix bug in circular singleton creation detection

Summary:
We considered it circular if we tried to create a singleton
while the singleton was being created.  In a single threaded world, this
is correct, but under concurrency, two threads can be in the singleton
creation codepath and become confused about the state of the singleton.

This change uses a condition variable to notify when creation completes
so that other threads can wait on the creation to complete.  Circular
creation is detected via thread id.

Test Plan:
runtests (new test case; failed without the fix, passes with
it)

Reviewed By: hans@fb.com

Subscribers: lins, anca, njormrod, rkroll

FB internal diff: D1534081