Implicitly change all the DBPrimary from the removed shard on a removeShard operation.

To do that we should call the _shardsvrDrainShard command from the _configsvrRemoveShard execution thread.

Considerations to be taken: 

• We can't call _shardsvrDrainShard unless all the checks to remove a shard pass.
• Config server will tell the draining shard through the _shardsvrDrainShard command which is the destination shard for all the databases.
• The balancer won't start moving the unsplittable collections related to the removeShard operation until their database is been moved off the draining shard.