Summary:
Improvements to the RocksJava release process:
* Generates the Maven artifact version number as part of the release step
* Also generates appropriate checksum files to speed the deploy and publish step

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7219

Reviewed By: ltamasi

Differential Revision: D22983481

Pulled By: pdillinger

fbshipit-source-id: 7b8ffaf46471cd3cda181eb830c962b317d2e688