Awaitable isMaster can be triggered via the use of topologyVersion and maxAwaitTimeMS fields. We should begin to use these isMasters with the ReplicaSetMonitor scanning. This does not require use of exhaust/tailable cursors.
The use of awaitable isMaster may need to be opt-in for testing until we have mongodden/mongossen that support this work. However, this work should be verified for correctness via modification of existing integration tests and js test suites.