[repoapi] introduce a basic ClientConnection class This is the new official way to access the repo from client side. It still access Session object directly as the server side connection is not up yet (and it's not up because it would have no user). Multiple follow up commit will install compatibility with the DBAPI. This will ease the migration from dbapi to repoapi. ClientConnection has no user yet but later commit will use it in the whole Web stack. related to #2503918

[req] add a _set_user method RequestSessionBase usally need to recreate a new CWuser appobject linked to themself. We add it to the base class to avoid multiple redifinition.

[server/session] add a login property session.login is a DBAPISession attribute. 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 #2503918

[server/session] allow access to session id using sessionid session.sessionid is a DBAPISession attribute. 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 #2503918

[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