[sources] ensure we have a cnxset in undoable_transactions

[server] move security/integrity hook management away from InternalSession.__init__ and to Repository.internal_{session,cnx} instead. Lets internal_cnx avoid deprecation warnings.

[migration] make 'session' object be a server-side Connection, not ClientConnection I think this better corresponds to the previous behaviour. However, that Connection does not have a cnxset, which should probably be changed (for both interactive_shell and cmd_process_script).