Werk #2899: ps: Finally improved performance impact of perfdata when having a lot of processes

When having a lot of processes, like several thousands, on a system and monitoring them, this lead to a lot of performance counters being created in Check_MK. The management of these counters was not very optimal, because not updated counters were only deleted based on time. So systems with a lot of process creations were handling many non needed counters during every check interval. This affected CPU and memory usage during checking such systems.

As we get the full process table, we have now changed the logic to automatically delete the counters of all processes that are not existant anymore. This reduces the number of counters significantly.

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