## 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
""" mail_followers holds the data related to the follow mechanism inside
        OpenERP. Partners can choose to follow documents (records) of any kind
        that inherits from mail.thread. Following documents allow to receive
        notifications for new messages.
        A subscription is characterized by:
            :param: res_model: model of the followed objects
            :param: res_id: ID of resource (may be 0 for every objects)
    """
    _name = 'mail.followers'
    _rec_name = 'partner_id'
    _log_access = False
    _description = 'Document Followers'
    _columns = {
        'res_model': fields.char('Related Document Model',
                        required=True, select=1,
                        help='Model of the followed resource'),
        'res_id': fields.integer('Related Document ID', select=1,
                        help='Id of the followed resource'),
        'partner_id': fields.many2one('res.partner', string='Related Partner',
                        ondelete='cascade', required=True, select=1),
        'subtype_ids': fields.many2many('mail.message.subtype', string='Subtype',
            help="Message subtypes followed, meaning subtypes that will be pushed onto the user's Wall."),
    }
    #
    
    
    #
    def create(self, cr, uid, vals, context=None):