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.