[server/session] Implement anonymous_session Now we have a simple rule to compute if a session is anonymous we can implement the property for server session too. Having it on server side session will helps the rework of the API to access repository. The new schema drop the concept of DBAPISession and use server side session for the same purpose. Related to #2953943 Related to #2503918

[server/session] do not clear session.cnx if unrelated to cnx actually cleared The cnx parameter may be different from the one actually loaded for the current thread. This will be possible in future commit when you close a client connection for example.

[dbapi] move ProgrammingError into cubicweb module A new ``repoapi`` will be introduced as a replacement for the dbapi. It will need ProgrammingError too. Related to #2503918

[repoapi] move get_repository function into a new repoapi module This new module aims to host the function of the new API replacing the old DBAPI. `get_repository` is still needed hence moved to the new module. Related to #2503918

[testlib] gather all repository access logic in one place Refactoring of the repository access API in test is imminent. We plan to move from the "old" dbapi to the new repoapi. Gathering all impacted method in one place help to understand how all those method interact and help readability for both patch and resulting code. No code change is done at all in this changeset. The refactoring will code later.

[testlib] move repo and related attribute back on Instance instead of Class The repo and cnx was hold by the TestCase class to work as a cache and avoid recreation of the repo from scratch of each test. However since bad26a22fe29 the caching is done by the DatabaseHandler object and it is not necessary have a second layer of cache on the TestCase itself. We move the repo and cnx attribute back on the TestCase instance itself and make several class methods instance methods again. This will helps to change dbapi access in test from dbapi to repoapi. related to #2503918

[dbapi] makes anonymous_connection a computed property The current implementation is a boolean flag set manually by client code after connection creation. This led to different way to decide a anonymous_connection should be True (eg. different in the test than in the actual application code). It should not be client responsibility to set this flag. ``cnx.anonymous_connection`` is now a purely computed property. Connection with user in the "guests" group are anonymous, the other ain't. ``Session.anonymous_session`` is computed from ``cnx.anonymous_connection`` and get the updated behavior transparently. Closes #2953943