BackgroundOperation currently tracks that an index build is active on a namespace, no other details. This is the current interface in the system to discover that index builds are running on a namespace, or to wait for index builds to finish on a namespace.

BackgroundOperation will be turned into an interface with which index builds can register themselves and that other operations can call into to affect ongoing builds. It will maintain an internal ThreadPool on which to run registered builds; as well as an internal data structure matching createIndexes command specs to an active builder. It should have function stubs for operations that external operations – like stepdown, dropIndexes – can call into to eventually affect ongoing index builds, like to notify of repl state changes or abort requests.