Adding in the ability to balance unsharded collections requires introducing new balancer modes. Currently the only mode available is kFull. We should add a few more modes:

• kSharded only - will allow sharded balancing and draining of unsharded collections when the user specifies a destination shard for removeShard
• kUnsharded only - will allow all types of unsharded balancing but not sharded balancing
• kDrainUnshardedOnly - will only allow draining of unsharded collections when the user specifies a destination shard for removeShard