step_down_during_draining.js should wait for writes to be durable before disable replication on secondaries using fail point

On slow machines, it may be possible that the durable optime has not caught up with the committed optime before we disable replication the secondaries. This may lead to failed elections when we request one of the secondaries to step up to become primary.

    replSet.awaitReplication();

    // Enable fail point to stop replication.

    var secondaries = replSet.getSecondaries();

    secondaries.forEach(enableFailPoint);

...

    stepUp(secondary);