bookmark: add a `bmstore.recordupdate` to plug bookmarks into the transaction Instead of manually writing bookmarks when they are updated, we can just record this update to the transaction and rely on it to update the on-disk file.

bookmarks: split bookmark serialization and file handling If we want to handle bookmarks in a transaction we need to decouple the file handling and the actual production of the content. This is similar to how we handle phases in transaction.