Batch rollup reads

https://github.com/rackerlabs/blueflood/pull/263
Merge pull request #268 from fourk/batch_rollup_reads2

Batch rollup reads
fix concurrency issues around iterating the keyset for batch reader
Merge pull request #269 from fourk/fix_concurrency_with_batch_reads

fix concurrency issues around iterating the keyset for batch reader
avoid concurrentmodificationexception
Merge pull request #270 from fourk/fix_concurrency_with_batch_reads

avoid concurrentmodificationexception