Add timeout as option to dataconnection

[DirkHeinke]

This will add a new option to the datachannel to enable a timeout function (#769).

This works by sending a special message every N seconds and automatically close the channel if the message from the other end was not received within N*2 seconds. The receiver does not emit the special message to the user.

This implementation works but nothing has been added to the documentation. After a maintainer confirms, that this is the right way to do it, I'll add documentation and tests.

[geniuskidkanyi]

Is this still under development?