Row format. Use varint for sizes.

We already compact row chunks after writing it and can ensure the row is always written in the most compact way.

It may be worth writing chunk length as Varint. At least for tiny chunks.

Saving just 1-2 bytes for mid-size rows (payload of 255->64k) may look not too beneficial,
but 3-6 bytes for tiny rows does (with payload size up to 255 bytes). E.g. when the key is Long.