## 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

return {
            purchase.id: {
                'shipment_count': len(purchase.picking_ids),
                'invoice_count': len(purchase.invoice_ids),                
            }
            for purchase in self.browse(cr, uid, ids, context=context)
        }
    STATE_SELECTION = [
        ('draft', 'Draft PO'),
        ('sent', 'RFQ'),
        ('bid', 'Bid Received'),
        ('confirmed', 'Waiting Approval'),
        ('approved', 'Purchase Confirmed'),
        ('except_picking', 'Shipping Exception'),
        ('except_invoice', 'Invoice Exception'),
        ('done', 'Done'),
        ('cancel', 'Cancelled')
    ]
    READONLY_STATES = {
        'confirmed': [('readonly', True)],
        'approved': [('readonly', True)],
        'done': [('readonly', True)]
    }
    _track = {
        'state': {
            'purchase.mt_rfq_confirmed': lambda self, cr, uid, obj, ctx=None: obj.state == 'confirmed',
            'purchase.mt_rfq_approved': lambda self, cr, uid, obj, ctx=None: obj.state == 'approved',
            'purchase.mt_rfq_done': lambda self, cr, uid, obj, ctx=None: obj.state == 'done',