[Cutover] Read sequence value and use that for updating during cutover

[levkk]

Description

Instead of getting MAX(id) from a table to set its sequence, use the sequence value on the original primary. Using MAX(id) doesn't account for deleted records and could end up re-using identifiers that have been stored in external systems.

[levkk]

We use unique IDs in favor of sequences in sharded DBs.