CS-Studio archiver: alarm states get "stuck"

## Where?

On EMU currently the block archiver believes that `field_hifi` is in `SCAN_ALARM`. While the PV *does* intermittently go into this alarm state, it seems that the archiver has not correctly cleared the alarm state. This causes the web dashboard to display an alarm when no alarm exists on the underlying PV.

I was not able to reproduce this locally - more extensive testing will be required as part of this ticket.

## How?

For background information, see https://github.com/ISISComputingGroup/IBEX/issues/6399 - in particular the later comments which document a concrete occurence of this problem on EMU.

## Reproducible?

No

### To Reproduce

- Unknown

## Acceptance criteria
- [ ] Archiver correctly picks up changes of alarm state in all cases

## How to Test
_verbose instructions for reviewer to test changes
(**Add before making a PR**)_

Start with the concrete EMU occurrence documented in issue 6399, then trace how the block archiver receives and clears PV alarm-state changes. The issue names no source files or tests, and reproduction is currently unknown. Done means verifying that alarm states are cleared correctly when the underlying PV leaves the alarm state.

Vom Indexierungsmodell aus dem Issue-Text verfasst.