Right now, establishShardCursors() returns RemoteCursors, which could cause cursors to leak if exceptions are thrown before callers have the chance to kill the cursors. The function should be modified to return OwnedRemoteCursors from the get-go so that no callers will need to worry about leaking cursors.

Some callers place raw RemoteCursors into objects like AsyncResultsMerger, which should also be modified to take in owned cursors.