AsyncResultsMerger's cursor cleanup is [fire and forget|
https://github.com/10gen/mongo/blob/2de947c5f21dd4b1e676823d92f065b1bd3e2c33/src/mongo/s/query/exec/async_results_merger.cpp#L1620], which means that it might end up leaving orphaned cursors if it hits some kind of error. This can leave resources in the server and in some cases even block processes (like range deleter).

The idea is to register cursors to be killed to a service that will make more effort to ensure that cursor no longer exists.