Fix or Remove Morton Order in Volume Data Fossil Keys

[jstriebel]

Currently the FossilDB keys for volume data contain the Morton Order number. This is currently unused and broken, because

• we do not write multiple buckets, so that the Morton order would help.
• the Morton order number in the key does not contain leading zeroes, making this useless.
• the number only has 32bit which is not enough (breaking around 1000 buckets per dimension). This also lead to negative numbers (leading to bug Download of small volume tracings produces 0 b data.zip #3540), why we are currently not using this part of the key anymore.

We should either remove the Morton order part of the key, or make it useful again at a point. This involves rewriting the database.