Randomize sentinel order per-run, so that not all redis-sentinel instances use the exact same sentinel.

The way that redis-sentinel uses the sentinels array, it is guaranteed that all instances of it will use the same sentinel at all times.  This is less than ideal -- preferably, if there are N sentinels alive then we want our clients to spread out over them to mitigate the impact of the loss of a sentinel.

If we randomize the sentinel array ordering during redis-sentinel initialization, that should have the desired effect.