This introduces djblets.db.query_comparator, which contains the query

comparison code from assertQueries(). This was moved out in order to

help make the code more maintainable, as it will soon need to grow in

size in order to handle ORM sub-expressions, and it was already pretty

large to begin with.
Since we're starting fresh, some renames were made. Instead of

"failures", we now describe results in terms of "mismatches", to help

differentiate from a query that fails to execute.
The inner functions have been made outer functions, avoiding the need to

redefine them when asserting.
Otherwise, nothing has really changed about the assertion logic. This

merely lays the groundwork for upcoming work on comparisons.
assertQueries() has been updated to use this, and has reduced in size

considerably.
Given some of the renames, this does break typing compatibility with

newer code, but not with anything that has been released. The affected

code will be updated to use the new type imports.

Unit tests pass.