Admin command to update the chunk metadata for only one collection

It would be helpful if there were an admin command on the mongos that would force the mongos to retrieve the newest chunk metadata for only one collection using the same method that is used when a StaleShardingException is caught for a given collection.

The existing flushRouterConfig() method flushes all of the chunk metadata, which then requires the mongos to retrieve the chunk metadata for every collection as requests for those collections are processed. This results in a much higher load on both the mongos and the config servers than the single-collection update process used when a StaleShardingException is caught.

This new method would be useful when an administrator moves a chunk or executes a movePrimary command and wants to be sure that all of the mongos instances have picked up the change without clearing out the metadata for all other collections, especially in larger clusters.