A typical use pattern for a framework scheduler is to remove its reservations before tearing itself down. However, it is racy: UNRESERVE is a multi-stage action which aborts if the framework is removed in-between.
Let schedulers use operation feedback and expect them to wait for an ack for UNRESERVE before they send TEARDOWN. Kind of science fiction with a timeline of O(months) and still possibilities for the race if a scheduler does not comply.
Serialize calls for schedulers. For example, we can chain handlers here onto per-Master::Framework process::Sequence. For that however, handlers must provide futures indicating when the processing of the call is finished, note that most handlers here return void.