[RocksJava] - BackupInfos & Restore-/BackupableDB enhancements

Summary:
- BackupableDB deleteBackup method
- BackupableDB purgeOldBackups bugfix
- BackupInfos now available in Restorable-/BackupableDB
- Extended BackupableDBTest to cover more of the currently implemented functionality.

Test Plan:
make rocksdbjava
make jtest

Differential Revision: https://reviews.facebook.net/D27027
[RocksJava] Integrated review comments

Parameter types for BackupID are now aligned to int.
Merge pull request #366 from fyrz/RocksJava-Backup-Restore-Improvements

[RocksJava] - BackupInfos & Restore-/BackupableDB enhancements