The index builder interface established is established in
This ticket will add a Threadpool and move all the instances of MultiIndexBlock (index builder class) that are all over the place behind the interface and running on the Threadpool.
We should be able to register index builds via the interface and then wait upon a condition variable to hear back on the Status result.
Keep in mind
SERVER-37644, which is to make index builds joinable via the createIndexes command. The condition variable setup must be such that we can have multiple waiters who can all hear back about the same result. Maybe an interface internal helper function to get something to wait upon for a Status result.