deprecate Synchronized::operator->
deprecate Synchronized::operator->

Summary:
Synchronized::operator-> is dangerous and leads to accidentally
acquiring locks or accidentally releasing them too soon. Add a
deprecated attribute that suggests using explicit locking instead.

Reviewed By: yfeldblum

Differential Revision: D18351232

fbshipit-source-id: 7bd3702bdd9b58dfa0fc7e8b8b636db9b5d7af8f

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