Automatically publish remaining desired state upon device connection

[thjaeckle]

As an enhancement of #696 all remaining desiredProperties state selectable for a thing after implementing #700 shall be returned whenever a device reconnects to its cloud backend. For Eclipse Hono this can be determined by the TTD value Hono sends for messages indicating how long a device is connected and may receive downlink messages.

Ditto's ConnectionStatus mapper already uses that information to automatically manage a feature ConnectionStatus as part of a thing.

Based in this information (when a device connected to Hono connects), the remaining desiredProperties of a Thing are sent to the device.