When performing a Tool.objects.get(...) or a self.tool query, we

attempt to populate a cache of known Tool objects and then look up in

that cache. Those entries very rarely change, and are frequently

queried, so we want to limit the database hits.
There was a regression in this caching. While debugging performance

regressions from the Django 1.11 port, I noticed that we were getting

repeated queries for the same tools. This happened because Q() objects

were being used in places (apparently from accessing repository.tool).

It's not clear yet whether this was an issue in Review Board 3.0 (and if

so, this will be backported).
This change fixes this up by making the caching logic a lot more

extensive. We now check for all sorts of variations of an ID lookup,

factoring in id/pk fields, pk=... and pk__exact=... keyword

arguments, and Q() queries.
Unit tests were added to catch caching regressions.

Unit tests pass.
Manually verified that this fixed multiple queries for the same Tool

in the profiling logs.