Recreate/update sensors based on edited keys.

- Delete synthetic sensors that have period or unit changed
  and create new synthetic sensors
- Copy metadata from original sensor
- Don't fill in upper and lower ts for new synthetic sensors
- Add tests
- Refresh original's sensor_metadata when recreating synthetic sensors
  to enable recalculations of new types/units.
Act on otfrom's comments.
Merge pull request #433 from MastodonC/feature/period-changing-fix

Recreate/update sensors based on edited keys.