SensorService: Local cache and synchronization

[muuki88]

[1] Map instead of List Use the id implement in #26 to store all ISensors in a Map

[2] Implement (un)bindSensorObserver Observer doesn't get added to the observers list

[3] Reduce doubled code Both createConverter() methods do the same. Implement a subset of methods for the equals part. May use loadSensorEntity

[4] Implement loadSensorEntity Based on the id in #26

[5] Rename (un)bindSensorProvider to (un)bindSensorObserver

[muuki88]

Synchronize with DB in bindSensor(ISensorService) method.

• Sensor entity not available -> create and persist Sensor entity
• Senro entity available -> do nothing