[IMP] mail: added headers field on mail.mail, allowing to store headers data when sending notification emails.
Merge pull request #691 from odoo-dev/master-mail-headers-tde

[IMP] mail: added headers field on mail.mail, allowing to store headers data when sending notification emails. This commit in master improves the commit 839815fdd44742663742b3506329d5b46cb3e3f7 (saas-5) that was using computed but not stored values.
[REF] mail, crm, hr_recruitment, project, project_issue: simplified reply-to of notification emails (now displaying 'YourCompany -docname- <email>' instead of 'Followers of ...'; added same_thread directly on the mail.message model to ease the reply_to and message-id computation; factorized a bit the reply-to generation for a batch of records; udpated tests accordingly.