firebase-ios-sdk

DispatchQueue delayed callback improvements + testing (#784)

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.