xds: add key-frame/i-frame concept to delta xDS

We should add the capability to signify a delta update as a "key-frame"/"i-frame" (as in video encoding). The idea here is the management server can indicate via a boolean that the client should flush all state and proceed to add any resources indicated in the delta update.

In many cases this will allow for a substantially simpler implementation of the management server in which delta updates can be best effort with a periodic re-sync to ensure there is no drift.

cc @markdroth @htuch @wgallagher @tomwans