builders: dashboard should return all work items for a builder

[bradfitz]

With the new VM-based builders in #9492 (which we'll also be using for Docker-based builds going forward), we can easily scale from idle to hundreds of VMs in no time.

It'd be nice if the dashboard could return all builds that need to be done for a given build key.

For instance, the openbsd-amd64-gce56 builder is currently catching up from zero at 1 build every ~15 minutes. There's no reason they couldn't all be going at once right now. It would also help when there's an rsc submit storm.

Even better would be one dashboard URL to return all items that need to be done for the current page, across all build types, so I don't need to poll N URLs.

Assigning to @adg, but maybe @minux is interested too.

I can do the coordinator side. I'd just rather somebody else do the build dashboard app side.

[bradfitz]

Per discussion with @adg, I'm going to add http://build.golang.org/?mode=json (similar to http://build.golang.org/?mode=failures, but json, and including everything, not just failures).