C++: synchronize dbschemes

[aibaars]

This PR synchronizes the common dbscheme fragments used by C++ with other languages and adds C++ to the CI check that validates that all common fragments are identical across languages.

The most important changes to the dbscheme are:

• svn-related tables have been dropped; these are unused by any of our QL libraries and the svn extractor no longer exists.
• The type of the "file" column of the locations_default table is narrowed from @container to @file. As far as I know the extractor should never produce location information for "folders", so switching to @file and matching the dbscheme of all other languages felt the right way to go. If there is a good reason to allow @container locations then we can also adjust all other languages.

TODO: add mostly trivial up and downgrade scripts. The only relevant change is deleting svn tables when upgrading.