The balancer needs to support new functionality which will be required for the algorithm in SERVER-58471

After putting the balancer into a "merge chunks" mode via the per-collection command developed as part of SERVER-58270 ,
the balancer needs to work in a special mode where we attempt to merge and move chunks to shards where they can be merged.

The end goal is to reach a optimal number of chunks based on the default chunk size and the size of the data.
After the merge chunks algorithm signals convergence, the balancer needs to move back into the default mode.

• Estimate chunk sizes by using the dataSize estimation API
• Waiting for chunk size estimation should not completely block balancer, if sizes for chunks are unknown we need to queue the estimation operation and then move on to the next operation
• Store chunk sizes in config.chunk documents for later processing stages
• Balancer needs to perform merges operations which the merge algorithm decided on

See Kal’s script for proof of concept

Below a schematic of the new balancer round