## 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
'name': 'Attach2', 'datas_fname': 'Attach2',
            'datas': 'bWlncmF0aW9uIHRlc3Q=',
            'res_model': 'mail.compose.message', 'res_id': 0})
        attach3_id = self.ir_attachment.create(cr, user_raoul.id, {
            'name': 'Attach3', 'datas_fname': 'Attach3',
            'datas': 'bWlncmF0aW9uIHRlc3Q=',
            'res_model': 'mail.compose.message', 'res_id': 0})
        # 5 
        _subject = 'Pigs'
        _mail_subject = 'Re: %s' % (group_pigs.name)
        _body1 = '<p>Pigs rules</p>'
        _body2 = '<html>Pigs rocks</html>'
        _attachments = [
            ('List1', 'My first attachment'),
            ('List2', 'My second attachment')
        ]
        
        # CA
        
        # Da
        self.registry('ir.config_parameter').set_param(self.cr, self.uid, 'mail.catchall.domain', 'schlouby.fr')
        # Da
        self.mail_group.write(cr, uid, [self.group_pigs_id], {'name': '"Pigs" !ù $%-'})
        # Do: 
        new_follower_ids = [self.partner_raoul_id]
        group_pigs.message_subscribe(new_follower_ids)
        # Tes
        group_fids = [follower.id for follower in group_pigs.message_follower_ids]
        test_fids = new_follower_ids + [self.partner_admin_id]
        self.assertEqual(set(test_fids), set(group_fids),