Commits to Bitbucket Server repositories should not be lost due to build concurrency limits

As mentioned in discussion around BAM-15449 it is currently possible for commits to Bitbucket Server repositories to go unbuilt, due to concurrency settings on the plan. The scenario is:

• Bitbucket Server sends an event to Bamboo
• Bamboo receives the event
• Bamboo starts to queue a build but
• Bamboo's plan concurrency check notes that the build is already running, and stops queuing the new build.

This can be worked around by increasing the plan concurrency setting.