As we move index builds behind the new interface, concurrent metadata operations must start checking the Coordinator for in progress index builds, to cover concurrency issues where the Coordinator does not immediately instantiate a builder that then instantiates a BackgroundOperation to block concurrent metadata operations.

The BackgroundOperation class will remain until everything is fully switched over to the new system. The IndexBuildsCoordinator has already implemented parallel checks to BackgroundOperation, just need to start using them and link the interface in all the places required.