[Java] Add purgeOldBackups API

Summary:
1. Check status of CreateNewBackup. If status is not OK, then throw.
2. Add purgeOldBackups API

Test Plan:
make test
make sample

Reviewers: haobo, sdong, zzbennett, swapnilghike, yhchiang

Reviewed By: yhchiang

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D21753