Now, all messages can be executed 6 times by default (or more in case of
ConflictError), regardless their initial priority.
For compatibility reasons, the 'priority' column is still increased.

git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@32877 20353a03-c40f-0410-a6d1-a30d3c3de9de