[server] make internal_cnx get and keep a connection to the db - internal_session worked that way - it's less surprising for users if their internal connections "just work", rather than having to deal with the cnxset when using low-level APIs Includes a backout of 3f62606c01a2 "[repo] Fix register_user" which is no longer needed. Depend on logilab-database 1.12.1 to avoid errors due to pointless check in python's sqlite3 binding.

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