## Impacted versions:
 8.0 / 9.0 / 10.0
## Steps to reproduce:
* see https://github.com/odoo/odoo/issues/11841
* in a shell
  1. create an in memory record `order = self.env['purcahse.order'].new({....})`
  2. create an in memory record with a reference to the first one `order_line = self.env['purchase.order.line'].new({'order_id': order, ....})`
  3. order_line.order_id is an in memory recordset
  4. create a record for a model that inherits from mail.thread `self.env['product.product'].create({....})`
  5. At this stage order_line.order_id is an empty recordset

## Current behavior:
 In memory records are dropped when a call is done to self.invalidate_cache
## Expected behavior:
In memory records are preserved when a call is done to self.invalidate_cache


There are lot of places where the method `invalidate_cache` is called and the silent removal of the in memory records from all environments has unpredictable effects on fields modified by a computed method or an onchange or even related fields .... Moreover since all caches are invalidated each time you create / write / unlink  (https://github.com/odoo/odoo/blob/10.0/addons/mail/models/mail_followers.py#L111) an instance of a model that inherits from 'mail.thread', the performances of the server are severely penalized.

OPW703903

force_send=False, user_signature=True):
        """ Send by email the notification depending on the user preferences
            :param list partners_to_notify: optional list of partner ids restricting
                the notifications to process
            :param bool force_send: if True, the generated mail.mail is
                immediately sent after being created, as if the scheduler
                was executed for this message only.
            :param bool user_signature: if True, the generated mail.mail body is
                the body of the related mail.message with the author's signature
        """
        notif_ids = self.search(cr, SUPERUSER_ID, [('message_id', '=', message_id), ('partner_id', 'in', partners_to_notify)], context=context)
        
        new_notif_ids = self.update_message_notification(cr, SUPERUSER_ID, notif_ids, message_id, partners_to_notify, context=context)
        
        if context and context.get('mail_notify_noemail'):
            return True
        
        self._notify_email(cr, SUPERUSER_ID, new_notif_ids, message_id, force_send, user_signature, context=context)