## 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
"""Create the purchase order from the procurement, using
           the provided field values, after adding the given purchase
           order line in the purchase order.
           :params procurement: the procurement object generating the purchase order
           :params dict po_vals: field values for the new purchase order (the
                                 ``order_line`` field will be overwritten with one
                                 single line, as passed in ``line_vals``).
           :params dict line_vals: field values of the single purchase order line that
                                   the purchase order will contain.
           :return: id of the newly created purchase order
           :rtype: int
        """
        po_vals.update({'order_line': [(0,0,line_vals)]})
        return self.pool.get('purchase.order').create(cr, uid, po_vals, context=context)
    def _get_purchase_schedule_date(self, cr, uid, procurement, company, context=None):
        """Return the datetime value to use as Schedule Date (``date_planned``) for the
           Purchase Order Lines created to satisfy the given procurement.
           :param browse_record procurement: the procurement for which a PO will be created.
           :param browse_report company: the company to which the new PO will belong to.
           :rtype: datetime
           :return: the desired Schedule Date for the PO lines
        """
        procurement_date_planned = datetime.strptime(procurement.date_planned, DEFAULT_SERVER_DATETIME_FORMAT)
        schedule_date = (procurement_date_planned - relativedelta(days=company.po_lead))
        return schedule_date
    def _get_purchase_order_date(self, cr, uid, procurement, company, schedule_date, context=None):