Support ALTER TABLE t REORGANIZE PARTITION <list of partitions> INTO (<list of partition definitions>)

[mjonss]

Enhancement

See pingcap/tidb#38535 for more info about the feature.

TiFlash needs support for intermediate partitions, during the partition data reorg, where new the data from the reorganized partitions are copied to intermediate partitions, that will finally replace the original reorganized partitions.

This means that there will be multiple partitions that are active in TiFlash during the operation, since there will be possible to read from both the new and the old depending on schema version of the client.