fix: concurrent concurrent Merkle tree subtree updates

[ananas-block]

Issue:

• The Concurrent Merkle trees prior logic to update the subtrees after an update didn't cover all edge cases.

Solution:

• Filled subtrees are the Merkle proof of the the leaf at next_index, the first zero bytes leaf that has not been updated yet.
• Thus, we can use the changelog method update proof to update the filled subtrees with the changes from the latest changelog.

Changes:

• replace update subtrees with update proof

Note:

• it is not necessary to handle the case leaf_index == changelog_entry.index because it is not possible to update self.next_index with the update method