Improve reading out of OEM information from devices:
- requeue OEM readout tasks upon ds485 failures
- abort OEM readout tasks for dSMs and devices that are no longer present (will be requeued when they reappear)
- improve handling of OEM state, an incomplete LOADING will be remembered and loaded as UNKNOWN to force a new readout
implements #12854
@chhitz please have a closer look at this as well, and I think @remymahler also had his fun with the busscanner at some point, so Remy, you may want have to check it out too ;)