Don't throw in Singleton::get() if singleton is alive

Don't throw in Singleton::get() if singleton is alive

Summary:
This is a fix for situations where you know that something is keeping singleton alive (by getting a weak_ptr and locking it), and request a singleton instance via get() method (if e.g. it's hard to pass a pointer to singleton instance from a method which locked it). If shutdown was scheduled - an exception was previously thrown even though the object was not destroyed yet.

This simplifies conversion of legacy code to folly::Singleton.

Test Plan:
fbconfig -r gatehouse/usersets/tests
fbmake runtests

Reviewed By: chip@fb.com

Subscribers: trunkagent, njormrod

FB internal diff: D1619311