Better prioritization for packages with lots of tests

[Martin Pitt]

Whenever a package like glibc or perl gets uploaded, these trigger thousands of tests that fill the queues for several days. This can't (solely) be addressed by adding more capacity due to the sheer size. The main annoyance of this is that this blocks propagation of every other package in -proposed during that time.

AMQP has no concept of priorities within a queue (they are strictly FIFO), but what we can do is to add new debci-$release-huge-$arch queues:

 * The workers connect to these new queues in the same way as the regular debci-$release-$arch ones, so that each worker handles the regular and huge queues in a round-robin fashion.

 * britney knows how many tests it triggers for an excuse; if it's more than 20 (strawman), put them into the huge queue instead.

This is relatively simple and unintrusive and should get us what we want.

[Martin Pitt (pitti)]

infra side is trivial: https://git.launchpad.net/~ubuntu-release/+git/autopkgtest-cloud/commit/?id=c2bb5d76a

http://autopkgtest.ubuntu.com/running now shows the "huge" queues.

[Martin Pitt (pitti)]

britney side of this: https://git.launchpad.net/~ubuntu-release/britney/+git/britney2-ubuntu/commit/?id=8f1a646