Make large 2D histograms in monitoring_hists optional

[aaust]

REST production on the OSG has very different requirements for computing resources than the production on HPC farms. Ideally, a single-theaded job occupies less than 4GB of RAM. A large fraction of the memory is occupied by 2-dimensional histograms in the monitoring_hists plugin, which may not be needed for every job. We should implement flags to make some of the histograms optional and benchmark the typical memory footprint.