CloudBees Replication Parent 2576

When a controller attempts to load a Pipeline build from disk but fails to reconstruct its flow graph (list of stages and steps), normally it saves a copy of the original metadata and then replaces it with a placeholder indicating that the original was broken and cannot be used. However there have been some cases in HA controllers where various timing-dependent issues caused replicas to temporarily observe inconsistent metadata, and the attempts to clean up and swap in the placeholder have in some cases resulted in worse problems than this system was intending to solve. Therefore the placeholder system is now disabled by default in HA controllers; any persistent metadata corruption in a build will just be displayed as such whenever the build is viewed, without attempting to make further changes on disk.