applyChange abstract method
Apply a change to a document.
Returns true if the change was applied, false if it was a duplicate
(already known).
Implementations MUST let CausallyNotReadyException propagate when the
change depends on operations the document does not have (and that are not
covered by a snapshot). The server relies on this to detect an
out-of-sync client and trigger a re-sync; swallowing it would silently
drop the change.
Implementation
Future<bool> applyChange(String documentId, Change change);