[Port of https://github.com/firebase/firebase-js-sdk/commit/0fa319e5e019dd0d40ab441d2ff9f8f6d4724e43]
* Refactored FSTOnlineState tracking out of FSTRemoteStore and into new
FSTOnlineStateTracker component.
* Added a 10 second timeout to transition from OnlineState.Unknown to
OnlineState.Offline rather than waiting indefinitely for the stream to
succeed or fail.
* Removed hack to run SpecTests using an FSTDispatchQueue that wrapped
dispatch_get_main_queue(). This was incompatible with [FSTDispatchQueue
runDelayedCallbacksUntil:] since it queues work and blocks waiting for
it to complete. Now spec tests create / use a proper FSTDispatchQueue.
* Added a SpecTest to verify OnlineState timeout behavior.
* Misc cleanup:
* Renamed FSTOnlineState states: Failed => Offline, Healthy => Online
* Renamed FSTTimerIds (ListenStreamConnection => ListenStreamConnectionBackoff)
* Added ability to run timers from spec tests.
* Move all Firestore files to Objective-C++
* Update project file references
* Don't use module imports from Objective-C++
* Use extern "C" for C-accessible globals
* Work around more stringent type checking in Objective-C++
* NSMutableDictionary ivars aren't implicitly casted to NSDictionary
* FSTMaybeDocument callback can't be passed a function that accepts
FSTDocument
* NSComparisonResult can't be multiplied by -1 without casting
* Add a #include <inttypes.h> where needed
* Avoid using C++ keywords as variables
* Remove #if __cplusplus guards
Michael Lehenbauer