Cancel all user operations on heartbeat stepdown path

Currently we cancel all running operations associated with an incoming connection when we receive the replSetStepDown command, but we don't do the same for when we step down due to our liveness information. We should also cancel ongoing operations for those stepdowns, to A) bring the behavior of the two stepdown paths into alignment and B) protect against an operation that starts when we are primary, yields, we step down, then step back, then the operation resumes.