Replication Backup creation fails with NPE, while deleting the old backups

in Solr back up SnapShooter will try to delete the old backups saved on the disk automatically, but deletion fails as the files may not be the pattern type expected by the OldBackUpDirectory.

```
private static final Pattern dirNamePattern = Pattern.compile("^snapshot[.](.*)$");
```
In this case the following code throws NPE.

```
if (obd.getTimestamp().isPresent())

```

also in OldBackUpDirectory`s pattern match should haver matcher.matches(), to fetch the group(1) value.