firebase-ios-sdk

Identify limbo-only document updates (#1167)

* Filter out document updates from target association changes

* Move remote-event-modifying methods onto remote event

* Style

* keep limbo docs in remote event

* Implement identification of limbo document updates

* Refactor a bit to handle removes as well

* Drop newline

* Handle synthesized limbo deletes

* Response to feedback

* Appease the style gods