erp5_interface_post: use a serialization_tag per mail reference when sending them

To prevent race-condition in a multi-node environement (ie: 2 activity nodes load the afterAllowExport activities for 2 Posts having the same reference)

bt5/erp5_interface_post/SkinTemplateItem/portal_skins/erp5_interface_post/InterfacePost_afterAllowExport.py

@@ -6,4 +6,8 @@
 # to send it by bunches using alarm. Or ...
 
 # Priority 2 is just enough to be executed after the immediateReindexObject
-context.activate(priority=2).InterfacePost_disallowExportOfSameInterfacePostListAndExport()
+tag="InterfacePostWorkflow_afterAllowExport:%s" % context.getReference()
+context.activate(
+  serialization_tag=tag,
+  priority=2,
+).InterfacePost_disallowExportOfSameInterfacePostListAndExport()