SnapshotFileCache won't update for snapshots stored in S3

The SnapshotFileCache depends on last modified time to determine whether to update the Snapshot HFile cache. However, in S3, real 'folders' don't exist. S3 filesystems create a dummy file in place of a folder, but the dummy file last modified time is not updated when files are changed 'under' it. This means that the SnapshotFileCache doesn't pick up new snapshot HFiles and these files aren't removed from the HFileCleaner and can be eligible for deletion.

My patch removes the lastmodified assumption.