DNS round-robin support

It looks like the driver now has the framework to support multiple connections to shards in-place, so I would like to suggest allowing "seeding" the shard router with one DNS record that references multiple addresses leading to having multiple mongos connections for both load balancing and failover. While I guess (not tested yet), that specifying several different address might do the trick, having it use the ability of DNS to have multiple addresses for one name would be very good for administration purposes.