[FIX] mrp: cancelling and validating impact on planning

The commit 0679d0d5 introduced a new
planning mechanism for the workorder. Two issues have been instroduced
since this patch.
  1. Cancelling a workorder didn't free the time slot in the workcenter
    calendar
  2. Validation a workorder sooner than the planned date didn't update
    the workcenter calendar to plan another production sooner.

This commit update the date_planned_start and date_planned_finished
at cancellation and validation to impact the workcenters calendar.
To keep the original planning dates, they are now tracked

This commit also fix the button_start() method that always rewrited the
date_start even if the production had already been started.

Task : 2082442

closes odoo/odoo#38947

X-original-commit: 4ca6c773
Signed-off-by: Simon Lejeune (sle) <sle@openerp.com>