Summary:
Added the following statistics to BackupableDB:
1. Number of successful and failed backups in class BackupStatistics
2. Time taken to do a backup
3. Number of files in a backup
1 is implemented in the BackupStatistics class
2 and 3 are added in the BackupMeta and BackupInfo class
Test Plan:
1 can be tested using BackupStatistics::ToString(),
2 and 3 can be tested in the BackupInfo class
Reviewers: sdong, igor2, ljin, igor
Reviewed By: igor
Differential Revision: https://reviews.facebook.net/D22785
6cc12860f0