firebase-ios-sdk

Offline get() improvements. (#1783)

[Port of https://github.com/firebase/firebase-js-sdk/pull/1197]

1. Changed MAX_WATCH_STREAM_FAILURES to 1 per conversation with @wilhuff and
   @jdimond.
2. Fixed a "race" where when a get() triggered a watch stream error, we'd
   restart the stream, then get() would remove its listener, and so we had no
   listener left once the watch stream was "opened". Without a listen to send
   we'd be stuck in Offline state (even though we should be Unknown whenever
   we have no listens to send), resulting in the next get() returning data
   from cache without even attempting to reach the backend.