Versioning and Rocksdb 6+

[jamesbibby]

We are still stuck on version 5 of rocksdb and I would like to be able to support some of the new features in version 6.

I propose that we take a branch of current master named rocksdb5 and update the docs so folks can still build and run against version 5.

We can then start updating master to expect rocksdb6 or higher.

Any thoughts/concerns on the approach?

[hkparker]

Are we just missing features with 6, or will building with 6 cause issues? I built gorocksdb with the master of rocksdb today and everything seemed to work, but if this unstable it would be good to know.