next/jdbc: migration id should be unique

[holyjak]

IMO it makes sense that the ragtime_migrations table's id column is unique because we most certainly do not want to apply the same migration twice.

[weavejester]

Thanks for the PR, and apologies for the delay in looking at this. I believe the original idea was to keep the table creation SQL minimal for maximum compatibility, but I'm not aware of any SQL database that lacks primary keys, so this seems a reasonable change.

This change will need to be added to the ragtime.jdbc subproject as well to keep things consistent. Can you also change the commit message to:

Make id column of migrations table a primary key

[holyjak]

@weavejester done

[weavejester]

Merged manually.