Add a check_dbversion() method to the Db class

This method compares the version stored in the database in the `dbversion`
table with the currently declared version.

This current version is declared as a simple `current_version` attribute
on the Db class. **It must be updated jointly in 30-swh-schema.sql** when
modifying the db schema. Note that if this is forgotten, the added test
`test_dbversion` should fail.

Related to T2525.