[IMP] mail: add missing index

Without this index, deletion of mail.message records takes too much time

addons/mail/models/mail_mail.py

@@ -25,7 +25,7 @@ class MailMail(models.Model):
     _rec_name = 'subject'
 
     # content
-    mail_message_id = fields.Many2one('mail.message', 'Message', required=True, ondelete='cascade', auto_join=True)
+    mail_message_id = fields.Many2one('mail.message', 'Message', required=True, ondelete='cascade', index=True, auto_join=True)
     body_html = fields.Text('Rich-text Contents', help="Rich-text/HTML message")
     references = fields.Text('References', help='Message references, such as identifiers of previous messages', readonly=1)
     headers = fields.Text('Headers', copy=False)