Summary:
As per https://twitter.com/olebegemann/status/738656134731599872, our use of "main thread" to mean "main queue" seems to be unsafe.
This diff replaces the `NSThread.isMainQueue` checks with dispatch_get_specific(), which is the recommended approach.
I've also replaced all use of "MainThread" terminology with "MainQueue", and taken the opportunity to deprecate the "sync" param of `RCTExecuteOnMainThread()`, which, while we do still use it in a few places, is incredibly unsafe and shouldn't be encouraged.
Reviewed By: javache
Differential Revision: D3384910
fbshipit-source-id: ea7c216013372267b82eb25a38db5eb4cd46a089
72b363d7fc