[Java] Add purgeOldBackups API

Summary:
1. Check status of CreateNewBackup. If status is not OK, then throw.
2. Add purgeOldBackups API

make rocksdbjava
make sample

Reviewers: haobo, yhchiang, sdong, zzbennett, swapnilghike
Reviewed By: yhchiang
CC: leveldb

Differential Revision: https://reviews.facebook.net/D21753