Refactored Thing to return bridge UID instead of bridge object and completed support for persistence

* Enhanced StorageService interface to take ClassLoader as second argument.
* Using getService with ClassLoader in ManagedItemChannelLinkProvider, ManagedItemProvider and ManagedThingProvider.
* Added transient modifier to fields in domain classes, that must not be persisted.
* Moved VolatileStorage to test bundle and adapted tests.
* Removed StorageSelector and refactored managed provider to use one service only.

Signed-off-by: Dennis Nobel <d.nobel@external.telekom.de>