Add +[FIRServerValues increment:]
* Definition is currently in an "unreleased" extension to FIRServerValues
* Functions that resolve server values now need access to any existing snapshot; please help ensure that this is not done wastefully.
* Tests are added to FData.m
* Setting increment values
* Setting increment priorities
* Type coercion
* Overflows
* Some tests are renamed to make it clear that ServerValues != timestamps