Make addObserver/removeObserver take a write lock
Make addObserver/removeObserver take a write lock

Summary:
As title.  These methods look like they never protected observers_ list, and relied on
the user not calling addObserver or removeObserver concurrently.  I don't see any downsides
to taking a write lock here, these methods aren't called often.

Reviewed By: davidtgoldblatt

Differential Revision: D8187184

fbshipit-source-id: dd20b809e851cc14a05aca3bc9722e0a9236665f

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly