Add unique index for Asset/version/checkId in review table

[cd-rite]

putReviewsByAsset can sometimes create duplicate reviews when invoked concurrently. A unique index will at least prevent insertion of duplicate reviews, but will throw an error. This is better than creating the duplicate, but still sub-optimal.
Possible approach to avoid error:
lock asset for review updates when handling putReviewsByAsset

Adding index will require an operation in the migration cleaning up any existing duplicate reviews before applying index.

[csmig]

Resolved by #1416