This introduces djblets.db.query_catcher.catch_queries(), which

contains the query capturing logic from assertQueries(). Through this,

we now have a formal interface for query capturing that can be

maintained separately from assertQueries(), simplifying that logic

considerably.
With this being a utility function, it's now easier to introspect

queries during development and debug code without having to do the work

from within a unit test. This is very helpful when performance-tuning as

an alternative to checking django.db.connection.queries.
assertQueries() has been updated to use this.

All Djblets and Review Board unit tests pass.