System notification for unused CPU cores

I just finished a support call with a customer who had seemingly idle systems that were not able to process all messages being ingested and filling up the journal.

It was a 32 core machine (64 threads) running `graylog-server`, but the `*_processor` settings in `graylog-server.conf` were on the default setttings. The result was that only 4 of the available CPU threads were working on messages, with the remaining 60 threads being idle.

We should not expect customers to know this but we also should not just attempt to set the `*_processor` settings automatically because there might be other systems running on the same box and we'd be stealing resources from them, probably making the situation even worse.

This is why I suggest adding a new system notification that is triggered when less than `$cpu_threads-x` (x to be determined. this is the reserve for OS tasks, the garbage collector etc) are used by Graylog. The notification should link to a documentation page explaining the settings and why it is required. There should also be a way to disable the notification (in `graylog-server.conf`?) for setups where this condition is OK and the constant notification would be annoying.