## 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

addons/mail/tests/test_mail_features.py
'message_subscribe: incorrect followers after subscribe')
        # Do: 
        self._init_mock_build_email()
        msg1_id = self.mail_group.message_post(cr, user_raoul.id, self.group_pigs_id,
                            body=_body1, subject=_subject, partner_ids=[p_b_id, p_c_id],
                            attachment_ids=[attach1_id, attach2_id], attachments=_attachments,
                            type='comment', subtype='mt_comment')
        msg = self.mail_message.browse(cr, uid, msg1_id)
        msg_message_id = msg.message_id
        msg_pids = [partner.id for partner in msg.notified_partner_ids]
        msg_aids = [attach.id for attach in msg.attachment_ids]
        sent_emails = self._build_email_kwargs_list
        # Tes
        self.assertEqual(_subject, msg.subject, 'message_post: mail.message subject incorrect')
        self.assertEqual(_body1, msg.body, 'message_post: mail.message body incorrect')
        # Tes
        test_pids = set([self.partner_admin_id, p_b_id, p_c_id])
        self.assertEqual(test_pids, set(msg_pids), 'message_post: mail.message notified partners incorrect')
        # Tes
        test_aids = set([attach1_id, attach2_id])
        msg_attach_names = set([attach.name for attach in msg.attachment_ids])
        test_attach_names = set(['Attach1', 'Attach2', 'List1', 'List2'])
        self.assertEqual(len(msg_aids), 4,
                        'message_post: mail.message wrong number of attachments')
        self.assertEqual(msg_attach_names, test_attach_names,
                        'message_post: mail.message attachments incorrectly added')
        self.assertTrue(test_aids.issubset(set(msg_aids)),
                        'message_post: mail.message attachments duplicated')
        for attach in msg.attachment_ids:
            self.assertEqual(attach.res_model, 'mail.group',