This is prep work for replacing ClusterAggregate's use of Strategy::commandOp() with establishCursors().

This is because aggregation may need to know which shard a cursor resides on, in order to select a merging shard from the shards involved in the aggregation.

Note david.storch charlie.swanson, it seems plausible to choose any shard to do the merging, not just one that had a cursor established on it. Is it actually necessary to choose one that had a cursor established on it?