v6.0-beta.39

* Issue **#1140** : Add health check for proxy feed status url.

* Issue **#1138** : Stroom proxy now deletes empty repository directories based on creation time and depth first so that pruning empty directories is quicker and generally more successful.

* Issue **#1137** : Change proxy remote url health check to accept a 406 code as the feed will not be specified.

* Issue **#1135** : Data retention policies are now migrated to use `Type` and not `Stream Type`.