implement message buffer

[emiltin]

If a site tries to send messages during a disconnect, the messages must be buffered, and send when the connection is reestablished.

• status subscriptions stay active during a disconnect, and still generate messages that are kept in the buffer
• buffered status messages must have q=old
• when connecting, the buffered messages are send as the last part
• Command messages, Version messages, Watchdog messages and MessageAck are NOT buffered.

Spec states that a power outage must not cause the buffer to be lost. This would require that data is persisted on disk. I think we can wait with that part.

ref:
https://rsmp-nordic.github.io/rsmp_specifications/core/3.2/applicability/transport_of_data.html?highlight=buffer#communication-establishment-between-sites-and-supervision-system

https://rsmp-nordic.github.io/rsmp_specifications/core/3.2/applicability/transport_of_data.html?highlight=buffer#communication-disruption