* Add data migration for moving to VersionedCollapsingMergeTree
This has been tested locally and partially on staging. Still requires a bit of work to verify.
Verification query:
```
SELECT main._partition_id, tmp.count, main.count
FROM (
SELECT _partition_id, count() AS count
FROM sessions_v2_tmp_versioned
GROUP BY _partition_id
) AS tmp
FULL OUTER JOIN (
SELECT _partition_id, count() AS count
FROM sessions_v2
GROUP BY _partition_id
) AS main
ON (tmp._partition_id == main._partition_id)
ORDER BY main._partition_id
```
* Add an early exit to migration
* cluster? extract common code
