## 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
if vals.get('name','/')=='/':
            vals['name'] = self.pool.get('ir.sequence').get(cr, uid, 'purchase.order', context=context) or '/'
        context = dict(context or {}, mail_create_nolog=True)
        order =  super(purchase_order, self).create(cr, uid, vals, context=context)
        self.message_post(cr, uid, [order], body=_("RFQ created"), context=context)
        return order
    def unlink(self, cr, uid, ids, context=None):
        purchase_orders = self.read(cr, uid, ids, ['state'], context=context)
        unlink_ids = []
        for s in purchase_orders:
            if s['state'] in ['draft','cancel']:
                unlink_ids.append(s['id'])
            else:
                raise osv.except_osv(_('Invalid Action!'), _('In order to delete a purchase order, you must cancel it first.'))
        
        self.signal_workflow(cr, uid, unlink_ids, 'purchase_cancel')
        return super(purchase_order, self).unlink(cr, uid, unlink_ids, context=context)
    def copy(self, cr, uid, id, default=None, context=None):