[Disk Manager] queue new ready snapshots of configured folders for backup - #7501

…image is deleted

A chunk leaves the follower when it leaves chunk_blobs. Chunk IDs are never
reused, so a late write or delete of its object cannot hit a live chunk.

- The backup copy writes a follower_chunks row before it queues a chunk; the
  worker marks it copied. The last unref of a chunk moves its row to
  backup_chunk_delete_queue in the same transaction, and the regular
  dataplane.DeleteBackupChunks deletes the objects.
- dataplane.DeleteBackupSnapshotData waits until the snapshot is gone and
  deletes its chunk map.
- Deleting a snapshot moves its queued backup attempt to backup_delete_queue.
  snapshots.DeleteBackupMeta cancels that attempt, waits for it to end, then
  deletes the chunk map and meta.json. images.DeleteBackupMeta deletes the
  chunk map and meta.json of a deleted image.

… backup on deletion

Deleting a snapshot no longer cancels its backup: the deletion waits for the
running copy, as the copy holds the snapshot. The copy writes meta.json last,
while it holds the snapshot, so meta.json is never written after the
deletion removed it.

- dataplane.BackupSnapshotData gets the meta key and contents in its request
  and writes them after the chunk map. snapshots.BackupSnapshot and
  images.BackupImage no longer write meta.json themselves.
- backup_delete_queue no longer carries a backup ID, and
  snapshots.DeleteBackupMeta does not cancel the backup.

…ckup

Queue new ready snapshots of folders listed in SnapshotStorageBackupConfig.FolderIds for backup in the transaction that makes them ready, with the create task as the backup ID. Existing snapshots are queued by the snapshots backup admin command.