[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.
make LevelFileNumIterator and LevelFileIteratorState anonymous

Summary: No need to expose them in .h

Test Plan: make release

Reviewers: igor, yhchiang, rven, sdong

Reviewed By: sdong

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D27645
FlushProcess

Summary:
Abstract out FlushProcess and take it out of DBImpl.
This also includes taking DeletionState outside of DBImpl.

Currently this diff is only doing the refactoring. Future work includes:
1. Decoupling flush_process.cc, make it depend on less state
2. Write flush_process_test, which will mock out everything that FlushProcess depends on and test it in isolation

Test Plan: make check

Reviewers: rven, yhchiang, sdong, ljin

Reviewed By: ljin

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D27561
Merge pull request #366 from fyrz/RocksJava-Backup-Restore-Improvements

[RocksJava] - BackupInfos & Restore-/BackupableDB enhancements