Werk #13226: SNMP caching bug leading to UNKNOWN services

An SNMP related caching bug could lead to services in the state UNKNOWN. While this has only been observed affecting interface services, it could also affect other services in principle. While the services reported "Item not found in monitoring data", a service discovery would report the services to be OK, and not resolve the issue. (Users that have a version prior to this werk can resolve the issue by removing the monitoring sites folder var/check_mk/snmp_cache/HOSTNAME)

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