Add caching of queries in a text file.

This add a `--cache FILE` option which can be used to provide a JSON file with canned "query -> files" mapping. This will hopefully limit the amount of failures we have in IBs due to DAS glitches or downtimes, given the files used in IBs are known to be at CERN and they are known to be there by construction. A separate pull request will integrate this in `runTheMatrix.py`.