Fully shard identities

Milestone %18.6

• Prepare sharding key to be fully sharded -> !209796
  • create a NOT VALID foreign key constraint on the sharding key column
  • run a batched background migration to fix existing rows
• Verify the BBM to fix existing rows completed successfully
• Verify there are no rows left pointing to inexistent records

Milestone %18.8 (required stop)

Milestone %18.9

• Fully shard table
  • finalize the BBM that fixed existing rows
  • validate the foreign key constraint on the sharding key