Make it possible to dump the query classifier cache to a file

With maxctrl show qc_cache it is possible to get the contents of the query classifier cache. However, if the cache is large, the memory needed for doing that is quite significant.

It would be much better if the content would be written to a file, as that would not carry that risk. The maxctrl show qc_cache could still remain, but return just a few items as that is riskfree and still useful in development context.