Reduce DAS (--onlypost) and github (worker tasks) requests

For the former: save a tree "skeleton" file in the results directory (and use that in postprocessing)

As mentioned in !86 (merged) : by saving a single event from any NanoAOD to the results dir, making plots from the postprocess method is completely independent from DAS queries (or the input file storage / xrootd). If no such file is found the same strategy is followed as before.

Testing done: processed a single file in sequential mode, copied the skeleton file to the results dir of a completed job, and picked it up when running with --onlypost.

For the latter: few minor updates to the JEC/JER database cache.