database: Feature request: Only write to DB on minimal change

[msinn]

This issue is moved over from the core repo. The complete issue can be read over there: smarthomeNG/smarthome#285

To reduce superfluous data it would be nice if logging of values to the database could be prevented if not a minimal difference to the last logged value is reached.

Use cases:

Values sent by physical sensors (like temperature, brightness, distance or flow sensor) often wobble in short intervals between two values (e.g. 24.1°C and 24.2°C and back again).
Unfortunately not every sending device allows to prevent this.
Sometimes an exact value is needed for live operations but for logging less precission would be sufficient.