Benchmark and improve performance for Reports::Dynamic

[mwarin]

Gather here all performace fixes to related to the reporting platform, which was introduced in:
#289

After putting into production and running against the production MongoDB, I could tell that this was going to need some tuning. It is slow and takes up an awful lot of resources. I can think of 3 approaches:

1. Benchmark ruby code and see what can be easily improved there.
2. Add indexes to MongoDB where missing.
3. See what can be parallelized without too much fussing.

In production, I would like the following basic query to take less than 20m.
It's currently at 3h+.

dyna_rep = Reports::Dynamic.new(
  base: "holdings",
  decorations: ["holdings.ocn", "holdings.local_id"],
  restrictions: {"holdings.organization" => "ucmerced"},
)
dyna_rep.run

41575

[coveralls]

coverage: 94.973% (+0.02%) from 94.951% when pulling a866b41 on report-platform-performance into 7952406 on main.