## 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

default_group_subtypes = self.mail_message_subtype.search(cr, uid, [('default', '=', True), '|', ('res_model', '=', 'mail.group'), ('res_model', '=', False)])
        
        
        
        
        group_pigs.message_subscribe_users([user_raoul.id])
        group_pigs.refresh()
        
        follower_ids = [follower.id for follower in group_pigs.message_follower_ids]
        self.assertEqual(set(follower_ids), set([user_raoul.partner_id.id, user_admin.partner_id.id]),
                        'message_subscribe: Admin and Raoul should be the only 2 Pigs fans')
        
        fol_ids = self.mail_followers.search(cr, uid, [
                        ('res_model', '=', 'mail.group'),
                        ('res_id', '=', self.group_pigs_id),
                        ('partner_id', '=', user_raoul.partner_id.id)
                    ])
        fol_obj = self.mail_followers.browse(cr, uid, fol_ids)[0]
        fol_subtype_ids = set([subtype.id for subtype in fol_obj.subtype_ids])
        self.assertEqual(set(fol_subtype_ids), set(default_group_subtypes),
                        'message_subscribe: Raoul subscription subtypes are incorrect, should be all default ones')
        
        group_pigs.message_subscribe_users([user_raoul.id], subtype_ids=[mt_mg_nodef])
        
        follower_ids = [follower.id for follower in group_pigs.message_follower_ids]
        self.assertEqual(set(follower_ids), set([user_raoul.partner_id.id, user_admin.partner_id.id]),
                        'message_subscribe: Admin and Raoul should be the only 2 Pigs fans')
        
        fol_ids = self.mail_followers.search(cr, uid, [
                        ('res_model', '=', 'mail.group'),