More responsive recipe builds

SteveK has recently landed a change that at the same time makes our admins happy, and should give our recipe users more responsive builds.

Daily builds were previously kicked off at 23:00 UTC. A rather arbitrary time that put quite a load on the build farm as recipes became more popular.

The new change has our script to kick of daily recipe builds run much more often, and uses a cunning mix of magic and smarts to run the jobs (ok, not so much magic).

When new changes are pushed to Launchpad for branches that are used in source package recipes, the recipe is marked as dirty. Dirty recipes are candidates for daily builds. If a recipe has not been built into the daily build PPA within the last 24 hours a new recipe build job is created very soon after Launchpad notices the new changes. If there has been a build, then the new build job isn't created until 24 hours has past since the last build. Manual builds into other PPAs do not affect the daily build time check.

What this means is that if you have a daily build recipe, and tend to change the branch less often than every day, then when you do change it, the package is built much more quickly.