## 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/purchase/purchase.py
'''
        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):