Reorganize classes/re-architecture code

[jwodder]

WIP design document for a new code architecture

• Add a DandisetBackup class that takes care of all repository-wide operations for backing up a Dandiset

  • Primary attributes:
    • dandiset: RemoteDandiset
    • ds: AsyncDataset
    • manager: Manager
  • Merge all Syncer methods into a single method of this class
  • Move the following Datasetter methods to this class:
    • update_dandiset()
    • ensure_github_remote()
    • sync_dandiset()
    • part of update_github_metadata()?
    • tag_releases()
    • mkrelease()
    • backup_zarrs()
  • Make update_dandiset_metadata() a method of this class
  • Make async_assets() a method of this class
  • Make register_s3urls() a method of this class

• Add a ZarrBackup class for subsuming ZarrSyncer?

• Wrap this block in a function that takes a DandisetBackup, aiterassets() async iterator, tracker, & error_on_change and returns a structure containing Report, dm.last_timestamp, dm.need_add, & dm.zarrs

• Move the addurl methods & attributes of Downloader to an Addurl(?) class, an instance of which will be passed to Downloader on instantiation

• Add a Superdataset class that takes care of operations on the superdataset