## 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

'''
        This function returns an action that display existing picking orders of given purchase order ids.
        '''
        if context is None:
            context = {}
        mod_obj = self.pool.get('ir.model.data')
        dummy, action_id = tuple(mod_obj.get_object_reference(cr, uid, 'stock', 'action_picking_tree'))
        action = self.pool.get('ir.actions.act_window').read(cr, uid, action_id, context=context)
        pick_ids = []
        for po in self.browse(cr, uid, ids, context=context):
            pick_ids += [picking.id for picking in po.picking_ids]
        
        action['context'] = {}
        
        if len(pick_ids) > 1:
            action['domain'] = "[('id','in',[" + ','.join(map(str, pick_ids)) + "])]"
        else:
            res = mod_obj.get_object_reference(cr, uid, 'stock', 'view_picking_form')
            action['views'] = [(res and res[1] or False, 'form')]
            action['res_id'] = pick_ids and pick_ids[0] or False
        return action
    def wkf_approve_order(self, cr, uid, ids, context=None):