Execution of replSetStepDown can race with unconditional stepdown via heartbeat response

Successful executions of unconditional stepdowns supercede stepdown attempts in response to replSetStepDown, but the execution of the command will not be killed if it wasn't issued by a client. Nodes attempting a priority takeover in pv0 send this command to the primary. A possible solution might be to make TopologyCoordinator::prepareForStepDownAttempt return an error if a stepdown has already completed (in addition to the existing stepdown-in-progress check.)