Skarmark applicability performance

1. Added/updated profiler conduit method to get all units of a specific type from a repository.
2. Updated original find_applicable_units profiler plugin api to calculate_applicable_units according to the new design.
3. Removed ApplicabilityReport plugin model since it is not going to be needed anymore.

I suggest looking at the files themselves instead of diffs. Please review related https://github.com/pulp/pulp_rpm/pull/275 at the same time.