public var hasPendingWrites:Boolean
     Returns true if the snapshot contains the result of local writes (e.g. set() or update() calls)
     that have not yet been committed to the backend. If your listener has opted into metadata updates
     you will receive another snapshot with hasPendingWrites equal to NO once the writes have
     been committed to the backend.
     
public var isFromCache:Boolean
     Returns true if the snapshot was created from cached data rather than guaranteed up-to-date server
     data. If your listener has opted into metadata updates you will receive another snapshot
     with isFromCache equal to NO once
     the client has received up-to-date data from the backend.