## 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/mail_followers.py
""" Class holding notifications pushed to partners. Followers and partners
        added in 'contacts to notify' receive notifications. """
    _name = 'mail.notification'
    _rec_name = 'partner_id'
    _log_access = False
    _description = 'Notifications'
    _columns = {
        'partner_id': fields.many2one('res.partner', string='Contact',
                        ondelete='cascade', required=True, select=1),
        'is_read': fields.boolean('Read', select=1, oldname='read'),
        'starred': fields.boolean('Starred', select=1,
            help='Starred message that goes into the todo mailbox'),
        'message_id': fields.many2one('mail.message', string='Message',
                        ondelete='cascade', required=True, select=1),
    }
    _defaults = {
        'is_read': False,
        'starred': False,
    }
    def init(self, cr):