firebase-ios-sdk

[Feature launch] Re-add increment operator (#5149)

* Definition is currently in an "unreleased" extension to FIRServerValues
* Tests are added to FData.m (integration tests)
* Some tests are renamed to make it clear that ServerValues != timestamps
* Functions that resolve server values now need access to any existing snapshot. This is done with lazy evaluation as described below:

Based on fix https://github.com/firebase/firebase-js-sdk/pull/2499
to bug https://github.com/firebase/firebase-js-sdk/issues/2487

Micro-benchmarking showed that N writes in succession led to N^2
performance when calculating the resolved write tree for those
writes (thankfully only at the subpath in this SDK). Lazy evaluation
of current data matches the optimization in JS that mitigated a
20% performance regression.