persist and regenerate motor positions and fpaMoved dates

INSTRM-215 gave the mechanism, but we still need to handle the persistence and keys. In particular, we need to generate the keywords whenever the actor is restarted.

 When the actor is started or some parts of it are reloaded:

• load persisted (date, pos)
• read actual motor positions from the controller
• if persisted and actual positions match, declare positions ok (i.e. set home flags to OK) and generate motor and date keys. This will be the normal case.
• if actual positions are real (> 0) but do not match the persisted positions, scream and invalidate everything.
• for the case when actual positions are == 0 and so indicate a power-cycled controller, add a command to allow human to declare that persisted positions are good, and to set the controller positions from them.