Werk #2859: Check_MK Discovery/Inventory and HW/SW Inventory services are now always using caches (if available)

The Check_MK services "Check_MK Discovery", "Check_MK Inventory" and "Check_MK HW/SW Inventory" need data from the Check_MK agent on agent based hosts. These check were contacting the agent on their own on every execution which could lead to logwatch messages not being processed by the agent because they were already processed by the discovery/inventory checks.

We now set these checks to use the agent cache (if available and if recent enough). This makes the checks only contact the agent when neccessary. This should never happen when the host is being monitored regularly because the regular monitoring ensures that the cache is available and up-to-date.

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