Self-remove drones from server's table

[Ianselot]

Request description
If a drone was power-off it will continue to stay in the server's table. It is uncomfortable.

Describe the solution you'd like
The drone must be self removed from table after constant delay time without telemetry. Set the delay time as a parameter in server's config file.

[artem30801]

Problem is known and likely to be solved by tweaking sock - keepalive params
Besides, IMHO very strict dependace to the telemetry messages isn't going to end well

[goldarte]

Not tested on Raspberry

[artem30801]

Tested, fix confirmed