Summary:
There are internal users who open BackupEngine for writing new backups only, and they don't care whether old backups can be read or not. The condition `BackupableDBOptions::max_valid_backups_to_open == 0` should be supported (previously in df74b775e6e3034714bb75b42aa8f97e9a47dd81 I made the mistake of choosing 0 as a special value to disable the limit).
Closes https://github.com/facebook/rocksdb/pull/2819
Differential Revision: D5751599
Pulled By: ajkr
fbshipit-source-id: e73ac19eb5d756d6b68601eae8e43407ee4f2752
f5148ade10