SnapshotCleaner allows to delete referenced files

The SnapshotFileCache is checking the lastModificationTime() of the ".hbase-snapshot" directory.
But in this way "pending" snapshots created in temp are not considered.
since only the parent directory timestamp is updated.

so, if we create .hbase-snapshot/.tmp/SNAPSHOT_NAME the lastModification timestamp of .hbase-snapshot will not be updated

https://github.com/apache/hbase/blob/af62395ef3f7e2a8be2ab16d272e5245902d7864/hbase-server/src/main/java/org/apache/hadoop/hbase/master/snapshot/SnapshotFileCache.java#L197