[DISCUSS][2.0] FLIP-349: Move RocksDB statebackend classes to o.a.f.state.rocksdb package

[Chesnay Schepler]

To properly reflect the state of the rocksdb statebackend I propose to 
move all classes in the state-backend-rocksdb module under the classes 
to o.a.f.state.rocksdb package.

https://cwiki.apache.org/confluence/display/FLINK/FLIP-349%3A+Move+RocksDB+statebackend+classes+to+o.a.f.state.rocksdb+package