Exclude should prefer to move write hot shards first

One reason to exclude a server is because the server is under too much load, so the goal is to take write load away from the excluded server. However, exclude is going to move shard away from the excluded server in a random order, so it may move a lot of cold data before hitting the hot shards.

If we issue movements to the DD queue at a higher priority if the shard is write hot, we can prioritize doing those excludes before the cold ones.

Start by tracing the exclude workflow and how shard movements are issued to the DD queue. Determine where write-hot shards can be identified and prioritized over cold shards; done means excludes move write-hot shards first rather than in random order.

Written by the indexing model from the issue text.