[connection] transparent cnx_set handling Connection object while take cares of there cnxset themself (as dbapi connection does). The ``set_cnxset`` and ``free_cnxset`` operation are still available for backward compatibility purpose. The ``_auto_free_cnx_set`` is introduced to handle mixed usage. A new context manager ``connection.ensure_cnx_set`` is added for code that access ``cnx.cnxset`` directly and are not wrapped in any specific ``Connection`` method. A ``_with_cnx_set`` decorator is used on all Connection method that need a cnxset.

[MESSAGES CONTROL]
disable-msg = C0301

[VARIABLES]
# Enable / disable this checker
enable-variables = yes
additional-builtins = _, display_name

[BASIC]
required-attributes=
attr-rgx = (w|[a-z_][a-z0-9_]{2,30})
variable-rgx = (w|[a-z_][a-z0-9_]{2,30})$
argument-rgx = (w|[a-z_][a-z0-9_]{2,30})$

[TYPECHECK]
generated-members=debug,info,notice,warning,error,critical,exception