[FIX] mass_mailing: bounced statistics

A mass mailing can be considered both as bounced and opened. This can
occur with some email providers, such as Gmail. Indeed, Gmail sends
out-of-office replies to the `Return-Path` address, which is also used
for bounced emails. Therefore, it is not possible to determine
accurately if a message sent to to the `Return-Path` is a genuine
bounced email, or an out-of-office reply.

However, if the recipient opens the email, it is possible to determine
that the bounce was counted incorrectly. In this case, we empty the
`bounced` field. Similarly, we only write on the `bounced` filed if the
`opened` field is empty. This could occur if the recipient opens the
email before the out-of-office reply is received.

As a bonus, we also make sure to avoid updating the status if no route
was found. No route means that the email was bounced, so no reason to
set it as opened.

opw-1869957

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