When a collection is sharded, mongods should be notified by the mongos

This is somewhat of an optimization, as separate mongoses are not guaranteed to pick up the newly sharded collection at the same time, and there can be no data host changes until migrations occur (at which point the mongods will load the versions). However, it would be clearer if the mongods were notified, and would eliminate version checks for other mongoses with correct info about the current version.

Also should verify that a cursor opened on an unsharded collection is treated correctly if sharding occurs afterwards.