[FIX] point_of_sale: repair rescue session system
[FIX] point_of_sale: repair rescue session system

Rev 87a3fe92de43dd7fce7e3364a5ca8dbb417ca7f8 revamped the
rescue session system in an attempt to simplify it, but
created new problems.

The rescue system is designed for accepting POS orders
that belong to a POS session that is already closed.

After 87a3fe92de43dd7fce7e3364a5ca8dbb417ca7f8:

- The system tried to automatically reuse an existing session
that matches the properties of the closed one. This will often
come as a surprise to the POS users, with orders appearing
from nowhere in a live session, possibly invalidating the cash
control steps, etc. Using a separate new session with an obvious
"RESCUE" name is much better for users. They will be able to
review the rescued order separately as well.

- When no open session was available, the system tried to create
a new one (without the rescue flag). This could often fail
because existing sessions could conflict with the new one, completely
breaking the recovery. (E.g. a session from the same POS but
with a different user) The rescue flag was designed to avoid this
pitfall, allowing multiple extra rescue sessions to coexist with
live ones.

This patch restores the use of the RESCUE flag for recovery
sessions, and forces the creation of a new session for every
recovered session. Recovery sessions will also be labelled as
"RESCUE" now, making it things more obvious for end users.

dfang/odoo,bplancher/odoo,Elico-Corp/odoo_OCB,bplancher/odoo,Elico-Corp/odoo_OCB,laslabs/odoo,Elico-Corp/odoo_OCB,storm-computers/odoo,laslabs/odoo,laslabs/odoo,bplancher/odoo,ygol/odoo,dfang/odoo,dfang/odoo,storm-computers/odoo,ygol/odoo,dfang/odoo,ygol/odoo,Elico-Corp/odoo_OCB,storm-computers/odoo,bplancher/odoo,Elico-Corp/odoo_OCB,storm-computers/odoo,bplancher/odoo,hip-odoo/odoo,storm-computers/odoo,hip-odoo/odoo,ygol/odoo,laslabs/odoo,hip-odoo/odoo,laslabs/odoo,dfang/odoo,Elico-Corp/odoo_OCB,ygol/odoo,hip-odoo/odoo,bplancher/odoo,hip-odoo/odoo,dfang/odoo,ygol/odoo,storm-computers/odoo,ygol/odoo,laslabs/odoo,hip-odoo/odoo