Currently, when committing a change to the allowMigrations flag, after executing a transaction to change the collection metadata and bump one chunk, there is a refresh sent to all shards. This is done to make all the shards aware of the change, and abort any ongoing migration if necessary.

This is correct, but it applies the pull model, which we're trying to move out from. We should instead, leave the commit command to simply change the collection metadata, and then make coordinators trigger the refresh if necessary.