Allow to migrate checkpoints for all type of sidegrades

When performing a full migration from SegmentMK to DocumentMK, it'd be useful to migrate the checkpoints as well. This way we can avoid full reindexing which occurs after such migration. We can copy checkpoints as follows:

1. Get the oldest checkpoint on SegmentMK and migrate it as the DocMK head.
2. Checkpoint the current DocMK head.
3. Get the next SegmentMK checkpoint and apply the changes.
4. Checkpoint the current DocMK, etc.
5. As the last step apply the changes from the SegmentMK head.

Since the checkpoint ids will be different, we should modify the /:async properties.