This feature is to make migrations make the chunk smaller if it realized that it is too big.

Things to consider:

• Currently, we just scan the shard key index and retrieve the record Ids of the docs, so we can't actually tell how big the actual documents are until we start fetching the documents for the recipient shard.
• We will need to add extra filtering in the xferMods command to filter out documents that are no longer part of the smaller chunk range.