ReplicationCoordinatorImpl::_requestRemotePrimaryStepdown doesn't work

This generates a stepdown command which fails:

{ ok: 0.0, errmsg: "stepdown period must be longer than secondaryCatchUpPeriodSecs", code: 2 }

This is called by a member with a higher priority that is not currently the primary so it can have a change to do a priority takeover, and to be elected. The fact that this is currently broken doesn't affect the system because the primary will also step-down on its own when it sees a higher priority member that can be elected. Fixing this might improve the timeliness of the step-down and forthcoming election.