One of my customer reported me that a side effect was done two time upon MailetContainer execution.
What was not my surprise when writing integration tests counting executions, that they were right!
Tracking down the bug, I encountered that `MailImpl.duplicate` do not preserve state, hence processing resumes from ROOT processor (leaving the exchange).
MatcherSplitter should preserve the state of the matched mail.
Preserving several mails on the same exchange, we should not shut the exchange down before processing is finished. Doing so would result in following I did set up reference counting for this.