Allow build concurrency to be set against specific builds

Allow concurrency to be set/modified at a build level such that you can specify certain builds to be waited on. This will allow for multiple workflows through a single Bamboo instance. Agents wouldn't be consumed (dedicated) for a specific build(s) and this would lower the overall time builds with specific requirements would have to wait to be grabbed by an agent.

Use case:
Multiple commits are made in succession and trigger multiple of the same build. If an agent is dedicated then the builds would run in order, but once those builds are done the agent remains unavailable to other builds. If this feature existed builds could not have any agents dedicated, but know not to run if a previous build would be running concurrently.