At most 1 message for a given serialization tag can be validated (cf [28706]).
Respect priority/date/uid when validating only 1 message (cf [34632]).

git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@34841 20353a03-c40f-0410-a6d1-a30d3c3de9de