[IMP] mail, mass_mailing: clean mail body processing and mail sent post processing

Purpose is to lessen query count by avoiding to browse and/or fetch data when
not necessary. This commit does two main things

 * first one is to avoid checking for existing statistics in mass_mailing
   where only checking existing of a mass_mailing on a mail to send is
   sufficient. It allows to avoid fetching data from statistics table;
 * second one is to rewrite part of the post_process code called after sending
   emails. Using list comprehension and mailing_id existence on mail_mail
   allows to save several queries on standard post flows;

Most tests benefit from this code update. On overall community runbot about
3K queries are saved with those small optimizations.

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