Serialize builds

[aandrieiev]

The main change is here.

[msawka]

• i don't think the check time needs to be configurable
• rather than callback and recursively wait, let's update the event log (to say that there's a pending build before), wait, and then requeue the message with rabbitmq. that would free up the WO to process more.
• we need to be careful here. just because a Workflow came back as "pending", doesn't mean that another WO or Builder isn't already processing it. This should check if the earlier workflow is the current workflow id, and then continue processing. This should eliminate the need for WorkflowAPIStub.