Refactor task backend's low-level data access.

[isoos]

• An iteration over RFC: Options to SQL-migration. #8705.
• Uses extensions methods to create a short-lived wrapper object that allows DatastoreDB.tasks.lookupOrNull('pkg') calls, which should be easy to reason about and also replace with their SQL counterparts.
• Some transactional update methods (e.g. updateDependencyChanged or bumpPriority) are easy to refactor enclosing the transaction logic, but others (e.g. the trackPackage method) is not that easy, and kept it as-is for now.
• Separated the insert/update methods, not really sure if it is helpful, but at least we have a quick overview where we are updating/inserting the entities.

[isoos]

wdyt @jonasfj @sigurdm?