Fix extreme slowness in hydra-queue-runner

If there are builds in the queue that depend on another scheduled
build, then hydra-queue-runner will start the dependency first and
block the dependent builds.  This is implemented in
findBuildDependencyInQueue.  However, if there are tens of thousands
of such dependent builds, since each call to
findBuildDependencyInQueue may take a second or so, hydra-queue-runner
will spend hours just deciding which builds *not* to do.  Thus very
little progress is made.

So now, when a build is started, we immediately check which builds are
"blocked" by it (i.e. depend on it), and remove such builds from
consideration.