Support for device configuration updates at runtime #331

- Created downstream messages to inform gateway of creation/updates/deletion of DeviceConfigs (as well as the creation and deletion of reservations)
- Created a caching wrapper for the DeviceDB interface to be used on the portals REST API. The cache wrapper listens on the aforementioned downstream events for DeviceConfig updates to refresh itself whenever necessary.
- Reacting on update and deletion events for DeviceConfigs on the gateway by reconnecting (immediately if update, retry later if deleted)