Let discovery message declare heartbeat period

[jonnor]

Split out from #30

The participant itself should indicate how often it intends to send discovery messages. This enables them to for instance optimize for battery usage (maybe a microcontroller which mostly sleeps, wakes up once every 5 minutes to measure and send), or reponsiveness.
The coordinator then needs to respect these per-participant values when considering "liveliness".

Needs a new key to be defined in the discovery protocol