Allow a build queue to grow past the "Concurrent Builds" limit

Currently if you attempt to start a build while there are already the maximum number of builds running, you get an error.

It would be nice, at least for some use cases, to be able to queue the build, but not start it yet. I think I'm asking for a true "Concurrency limit", and the addition of a new "Queue limit".

This would be especially useful for integrations - e.g. Matt Watson's Review Build plugin. I want to start a build on my branch, but we already have 4 running. I'm probably going to forget to start it later. Or even if I do remember, I'll have to wait longer than I need to before merging my branch (because the build wasn't started as early as it could have been). This is causing context-switching for me.