Add Scatter/Gather BatchUpdates and BatchDeletes to ScatterGatherBuilder.

Some related code cleanup.

I did make a backwards compatible change to ScatterGatherBuilder.RequestInfo constructor.  I seriously doubt this will affect anyone out in the wild, and I dont think it makes sense for RequestInfo to take a basic Request when all ScatterGathers should separate into BatchRequests anyway.

RB=149304
BUGS=SI-616
R=jbetz,si-dev
A=jbetz