sharded explain should return shard names, not addresses

In sharded explain, we return the connection strings for each shard hit, not the shard name - this seems inconsistent with how we generally describe a sharded setup.

Suggest returning something like :

shards : { shard0001 :

{ host : <conn string>, other_fields_of_explain : <> }

, shard0002 :

{ ... }

}