Allow reprocessing a mail without consuming it

Today reprocessing enqueue emails then destroy them straight away.

This is unfortunate as a user might want to replay the reprocessing several time, eg if the processing consume the mail but the user wants to debug the processing behaviour.

Today trying to reprocess a mail say 5 time while adapting the behaviour of the reprocessing (ie debugging).

I propose adding a query parameter `consume=true|false` defaulting to true controllong wether the original mail should be deleted from the repository or not.