Merge "Refactor persister to handle multiple storage engines. Allow persister to...

Merge "Refactor persister to handle multiple storage engines. Allow persister to load and save configuration snapshots from/to multiple storage instances. Loading works by iterating list of storages backwards and pushing first non-empty response to netconf. This allows having a default (initial) configuration for the controller that will never be overwrittern. Saving configuration will be propagated to all storage engines except those configured as read only."