Make folly::Singleton's destruction happen earlier

Make folly::Singleton's destruction happen earlier

Summary: Schedule destroyInstances to be executed via atexit in registrationComplete. registrationComplete is called from main(), so this makes sure folly::Singleton's will be destoyed before all singletons constructed before main() (which should cover all third-party libraries' singletons).

Test Plan: unit test

Reviewed By: chip@fb.com

Subscribers: trunkagent, njormrod, folly-diffs@

FB internal diff: D1717963

Tasks: 5666654

Signature: t1:1717963:1417659478:c50c271d1786be75499565f6ab9c0a1a6f6f347d