Long term: deal with positioners with unexpected behaviours

[albireox]

Right now we have three main robot categories:

• Normal robots that behave and reply normally.
• Disabled robots that may not move correctly but otherwise reply properly and know where they are.
• Offline robots that are not connected to a daisy chain but physically in the wok.

A fourth category of robots that we don't currently manage, mostly because we have been able to avoid them, are positioners that are connected and reply but with invalid or unexpected messages. For example, a robot that feels its sync line always high will fail to initialise because SET_SPEED fails with an ALREAD_IN_MOTION error. So far we have dealt with these cases by disconnecting them so that they become offline robots, but that may not be always possible.

Ideally we would want to mark these robots as offline but without actually disconnecting them, and that should work for most commands but not for broadcasts right now. If we mark these robots as offline the code will not expect a reply from them, but they will reply to most broadcast commands leading to replies arriving after the command is already done.

Solutions are to stop using broadcasts altogether (except for START_TRAJECTORIES) or add a new category of offline_but_alive that we count for broadcasts but otherwise treat as offline.