Sharding balancer schedules multiple migrations with the same conflicting source or destination

The sharding balancer policy avoids producing multiple migrations for the same shard.

However, the policy runs per collection and doesn't save any state across collections. Because of this, if there are multiple collections which need balancing, it may produce overlapping shards.

There is no correctness problem with this, but it will cause useless ConflictingOperationInProgress errors to pollute the config server and shard's logs on each balancer round.