Lower zstd compression ratio for non-sealed leafs

[Rüdiger Klaehn]

zstd compression at high levels is not cheap. So it seems that it is only worth it for things that we will keep for a long time.

Non-sealed branches are usually kept around only for a short time. So it would be possible to significantly speed up write ops by just using a lower zstd level for non-sealed branches.

For leafs it is tricky, since we don't know whether a leaf will be sealed before starting to compress things. But on average you got more branches than leaves when doing an update, so doing this for branches will have most of the benefit.