firebase-ios-sdk

Fix issue that could cause offline get()s to wait up to 10 seconds. (#978)

Port of https://github.com/firebase/firebase-js-sdk/pull/592

FSTOnlineStateTracker was reverting to OnlineState Unknown on every stream attempt
rather than remaining Offline once the offline heuristic had been met (i.e. 2
stream failures or 10 seconds). This means that getDocument() requests made while
offline could be delayed up to 10 seconds each time (or until the next backoff
attempt failed).