For the first time, the config server needs to run a command (listCollections) against a shard that returns a cursor.

The alternatives are to use DBClient::query or link establishCursors() and the ARM/ClusterClientCursor onto mongod, which is an entire summer 2017 intern project.

We should replace the plumbing of ShardRemote::exhaustiveFind with establishCursors/ARM once they are linked into mongod, so that we can do a clean replace of the Fetcher with those with one change.