Fixed log messages and implemented unregistering of handlers when thing is removed.

Signed-off-by: Dennis Nobel <d.nobel@external.telekom.de>
Extracted StorageSelector, which provides logic for selecting the storage service with the highest service ranking, from ManagedItemProvider.

Signed-off-by: Dennis Nobel <d.nobel@external.telekom.de>
Refactored ManagedThingProvider to use StorageService.