Ref T9456. When we tell CircleCI to build something, it's going to give us back a build ID, like "123".
Later, it will make a call to us saying "I finished the build for build 123, here it is: ...".
We need to be able to go figure out what "build 123" is. There's no real way to do this right now. We can store it on a target, but we can't efficiently figure out which target it is on without loading every target and then looking at all of them, which would be really slow.
Add a basic indexed key-value table so we can write something like <circleci.build#123, PHID-HMBT-abcdeffe23f2> and figure out which target the build is talking about.