db.get_repoid should use a transaction

[spbnick]

To avoid race condition between checking if a repo ID exists and creating one, db.get_repoid should operate within a transaction. That, or just revert back to using "INSERT OR IGNORE" (which is atomic) since nothing at the moment uses the separate db.create_repoid function.

[spbnick]

A better alternative would be removing ID creation from get_repoid altogether, leaving only ID retrieval.