botlink: liveness check on tinybrain connection

[Terkwood]

Although tinybrain+systemd script correctly reconnects after a power outage, the botlink service appears to be unable to boot the dead connection.

We determined this because we saw that about 50% of moves were being answered after the power outage and reconnect occurred -- presumably because botlink was trying to send data to the dead instance ?!

Implement a liveness check and make sure botlink cleans up dead connections

See #463

[Terkwood]

This might help with debugging local dev issues such as #518