fix!: Ensure that MeasurementIndexShardId is non-negative

[SanjayVas]

BREAKING CHANGE: This is updating an existing Liquibase changeset, and will therefore require manual adjustments wherever it has been rolled out. This should only have rolled out to Halo test environments.

Closes #1862

[wfa-reviewable]

This change is 

[SanjayVas]

Statements to roll back previous schema change:

DROP INDEX MeasurementsByContinuationToken;

ALTER TABLE Measurements
DROP COLUMN MeasurementIndexShardId;

CREATE INDEX MeasurementsByContinuationToken ON Measurements(
  UpdateTime,
  ExternalComputationId,
  State
);

DELETE FROM DATABASECHANGELOG
WHERE
  id = '21'
  AND author = 'sanjayvas'
  AND filename = 'kingdom/spanner/shard-measurements-by-continuation-token.sql';

I've already run this on halo-cmm-head and halo-cmm-qa. It will need to be re-run if the bad schema update is re-applied, e.g. by the Kingdom internal server restarting.

[SanjayVas]

I re-ran the statements on halo-cmm-head as it had gotten redeployed without this change.