It is convenient to be able to queue up another build run by accessing the
"...?token=buildnow" URL. But in fact, that's a bit too convenient! When
there's a need to kick off a new build, it can happen that many team members at
the same time go and queue up builds and suddenly we have a very long build
queue all of which are unnecessary except the first one.

Combine this with a product that takes several hours to build and it is a problem..

We'd like to have a configuration option where for a given product we can set
the max queue length to a given number. We'd set it to 1, so at most one future
build can be queued at any given time. Our project takes long enough to build
that any number > 1 doesn't make much sense. Others projects might want to set a
different max, so a configurable value would be best.