Use consistent hashing for SqlBagOStuff servers

Also factor out consistent hash code from Redis and the new application
into a class called ArrayUtils. The name "ArrayUtils" is from
I0f4e7d7c, I expect that change will be rebased on top of this one at
some point.

Change-Id: I9375087f4d7a6e8e629d97bfb6b117d9cb7d1bcf