As ReplicaSetMonitor can be constructed without _executor set via ReplicaSetMonitor() or ReplicaSetMonitor(const SetStatePtr&) c-tors
there is no need to invariant(_executor) in the d-tor
As ReplicaSetMonitor can be constructed without _executor set via ReplicaSetMonitor() or ReplicaSetMonitor(const SetStatePtr&) c-tors
there is no need to invariant(_executor) in the d-tor