Missing `once` or `get` and determination when search finished

You provide method `on` that tracks changes in real time, but I am interested to do search on static data once and determine when all children are added. In Realtime database it is done following way: `.once('child_added', (v, nextKey) =>{ if (nextKey === null) allDataLoaded() }`

Start with the existing `on` method and the GeoFire query/listener API, then compare their behavior with Firebase Realtime Database's `once('child_added')` pattern described in the issue. Done means a static-data search can expose when all matching children have been added, with the API behavior and completion condition clearly defined.

Written by the indexing model from the issue text.