Import tasks can be resource-intensive and potentially abusive. Allow one import per tool type (or importer, perhaps) per project to run at simultaneously. This can be tracked via the
get/set_tool_data. We'll have to make sure that error handling [#6530] decrements the count when done. Probably best to track an integer rather than a bool, so that its easier to customize if some deployments of Allura want to allow N simultaneous imports.