[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.
Basically a port of https://github.com/firebase/firebase-js-sdk/commit/a1e346ff93c6cbcc0a1b3b33f0fbc3a7b66e7e12 and https://github.com/firebase/firebase-js-sdk/commit/fce4168309f42aa038125f39818fbf654b65b05f
* Introduces a DelayedCallback helper class in FSTDispatchQueue to encapsulate delayed callback logic.
* Adds cancellation support.
* Updates the idle timer in FSTStream to use new cancellation support.
* Adds a FSTTimerId enum for identifying delayed operations on the queue and uses it to identify our existing backoff and idle timers.
* Added containsDelayedCallback: and runDelayedCallbacksUntil: methods to FSTDispatchQueue which can be used from tests to check for the presence of a callback or to schedule them to run early.
* Removes FSTTestDispatchQueue and changes idle tests to use new test methods.
Michael Lehenbauer