Improve granularity of progress notification (smaller max_download_size)

[sync-by-unito]

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.

[sync-by-unito]

➤ Jonathan Reams commented:

[~brian.munkholm], the current download message threshold size is 1MB, but either way this is something the cloud team would have to fix. [~mpobrien], is this already a configurable parameter on the cloud side?

[sync-by-unito]

➤ Jonathan Reams commented:

[~brian.munkholm@mongodb.com], is this still an issue?

[sync-by-unito]

➤ Brian Munkholm commented:

Good question. I'm not certain. It was first reported in the Java issue mentioned dating back to 2018. So I guess the reference to log compaction is related to legacy sync.
But if I'm not mistaken progress listener doesn't really work in Flex Sync? Until that's supported, I don't think this is worth pursuing?

[sync-by-unito]

➤ Jonathan Reams commented:

We've definitely lowered the download message size to 1MB a long time ago and [~daniel.tabacaru@mongodb.com] has basically implemented fragmented download messages already. I think this is done already unless there's something I'm missing here.

[sync-by-unito]

➤ Jonathan Reams commented:

[~brian.munkholm@mongodb.com] and [~christian.melchior@mongodb.com] it's been over 30 days since there were any updates here. I think we've implemented or mitigated everything that needs to be done here, so I'm going to close this as gone away. If there is still outstanding work, feel free to re-open.