## 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
res={}
        purchase_obj=self.browse(cr, uid, ids, context=context)
        for purchase in purchase_obj:
            res[purchase.id] = False
            if purchase.order_line:
                min_date=purchase.order_line[0].date_planned
                for line in purchase.order_line:
                    if line.state == 'cancel':
                        continue
                    if line.date_planned < min_date:
                        min_date=line.date_planned
                res[purchase.id]=min_date
        return res
    def _invoiced_rate(self, cursor, user, ids, name, arg, context=None):
        res = {}
        for purchase in self.browse(cursor, user, ids, context=context):
            tot = 0.0
            for invoice in purchase.invoice_ids:
                if invoice.state not in ('draft','cancel'):
                    tot += invoice.amount_untaxed
            if purchase.amount_untaxed:
                res[purchase.id] = tot * 100.0 / purchase.amount_untaxed
            else:
                res[purchase.id] = 0.0
        return res
    def _shipped_rate(self, cr, uid, ids, name, arg, context=None):