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

Using maxctrl, it is now possible to dump the contents of the query classifier cache to a file.

maxctrl create qc_dump [--format (json|pretty_json|json_lines)] <path>

The path should refer to an existing directory on the host where MaxScale is running. If it is not absolute, then it is interpreted relative to the data directory of MaxScale. The output of the command is the full path of the file to be generated. Depending on the size of the cache, it may take a short while before the file is available. With --format the format of the generated file is specified; json produces json, pretty_json produces json that is formatted for reading and json_lines produces a file where each line is a separate json object.

Original description
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.