[RocksJava] Support for stored snapshots

Summary:
RocksDB supports two ways of saving snapshots. In
memory and on disk. The later was added with this
pull request to RocksJava.

Test Plan:

Reviewers:

Subscribers:
[RocksJava] Addressed comments in D28971
Merge pull request #398 from fyrz/RocksJava-CreateCheckPoint

[RocksJava] Support for stored snapshots