We have a large default value 16MiB for max_download_size due to log compaction.
However, that leads to lumpy progress notifications.

A user reported it here:
realm/realm-java#6005

We should consider one or more of:

1. A smaller default value.
2. Let users set their cloud parameters themselves.
3. A dynamic calculation of max_download_size. If the history is 10MB, we should probably set max_download_size to 100 KB rather than 16MB as an example. The server has this information.
4. Fragmented messages.