Change RocksDB segmentation to use column family instead of per instance

With ISPN-8905 we added in support for segmented stores as well as a trivial method to take an existing store and replicate it for each segment.

RocksDB however has a built in support for segmentation via what they call as column families. We should try these instead which should reduce file descriptor usage and possibly increase performance.

https://github.com/facebook/rocksdb/wiki/Column-Families
https://github.com/facebook/rocksdb/blob/master/java/samples/src/main/java/RocksDBColumnFamilySample.java