MySQL, SQLite, PostgreSQL and MSSQL done with transitional patches.
One additional duplicate index removed from PostgreSQL schema.
Having a PK is essential to do maintenance, specially on large tasks.
By not having a PK it is impossible to add it in a safe way if not done
directly on the master.
Having a PK means that we can easily change the PK into another one if
needed in the future. The ones we chose might not be the best ones, but
will allow us to get them changed.
Bug: T198811
Change-Id: I6b96a427687085c6c24bcd759c9739f81288b919
