TaskMatcher rewrite [very wip]

[dnr]

What changed?

• Rewrite TaskMatcher to use explicit queues (to be extended into priority queues).
• taskReader pushes all tasks into TaskMatcher, and loads more when the number outstanding is too low.

This is not quite ready to merge, and there's at least one significant issue left to fix, but it passes most functional tests. I'm just making a PR to refer to it.

Why?

• Ability to prioritize uniformly across all pending tasks including queries+nexus.
• Simpler architecture, no new component between taskReader and TaskMatcher.
• More separated concerns, e.g. "forwarding" is (mostly) in one place instead of spread around, "rate limiting" is in one place, etc.
• Easier to understand code, no more nested selects (this is very subjective of course).
• Maybe better performance (after optimizations).

How did you test it?

existing functional tests, need to update unit tests

Potential risks

lots of new code, may be new bugs