Stash triggers should be queued while build is running

When a plan is building, triggers resulting from new commits are not queued.
The result is that the new commits are not being build.

To reproduce:

1. Create a Bamboo plan which uses a Stash repo, and uses the default "Stash repository triggered" trigger.
2. Create a job that:
   1. Checks out the repo.
   2. Script task with "sleep 60" to make the build last about 60 seconds.
3. Commit and push some changes to the repo.
4. Bamboo is triggered by the repo change, and starts building.
5. While the build is still running, commit and push another change to the repo.
6. After the build finished, the second commit is never picked up by Bamboo.