Previously there was nothing to uniquely identify a row in the
databasechangelog table as the uniqueness constraint was removed by
Liquibase (see https://liquibase.jira.com/browse/CORE-1909). Not
having something to uniquely identify a row can cause issues for
database replication software. Since we only support PostgreSQL, MySQL
and H2, the composite key referenced in the above issue (ID, AUTHOR,
FILENAME) will work for us. This commit puts that uniqueness
constraint back in for users wanting to replicate the Metabase
application database.

Fixes #8266