Archive Engine: false PV alarms

The archive engine may not be properly monitoring alarms (property changes?), as it is seeing alarms on some PVs that are not actually in alarm.
For example, on http://dataweb.isis.rl.ac.uk/IbexDataweb/default.html?Instrument=emu the DAE Simulation mode is displayed as in (INVALID/UDF_ALARM), but when doing a “caget –a” it is not showing as in alarm.

### Acceptance Criteria
- Look into why the archive engine is reading false alarms
- Improve the monitoring of the archive engine to stop it from seeing false alarms in the future

### Useful links:
Previous ticket: https://github.com/ISISComputingGroup/IBEX/issues/5765
http://ndxemu:4812/group?name=INST
http://ndxemu:4813/group?name=DATAWEB

### Discussion about the issue:

"Is the web dashboard reading the details from the archive engine? If so, then it rules out gateways as the archive engine is local to the instrument. Maybe the archive engine is not monitoring alarms (property changes?). It may just monitor dbf_control which gives it alarm status and units for free when value changes. In the case of sim_mode it will come up as “0 undefined” and then have a real 0 written to it and become defined, but the archive engine may not see this as the value has not changed (still 0). Can the archive engine be requested to monitor for property changes?"
"Yes it’s reading from the archive engine. Not sure if it can be set up to monitor alarms better. We could monitor the SEVR separately, but that will be a pain. Either way, it needs more looking into."