[FIX] models: preserve order when merging recordsets

RecordSets were not preserving order when combined
with AND/OR set operators. Order was lost as well
through mapped().

This caused various recurring "heisenbugs" during
tests executions on runbot, specifically comming
in the test of stock_landed_costs.

Using OrderedSets instead of basic Sets to resolve
these operation avoids the indetermination.

This required an implementation of __or__ / __and__
for OrderedSet.