/reviewed-on nexedi/erp5!671

1. accept a list of converison server
2. retry in case of network error

only calculate exchange rate if necessary
use transaction cache

remove useless test_21
update test_20 with various packing list

multiple owner test case
multiple cancel packing list test
circular movement test

(cherry picked from commit 27126964)

Let's use reference_date_method_id on Trade Model Paths.

* Code clean up
* Remove hardcoded ledger to pick date
* Add code to defined source section when none is defined

* Fix for special case where purchase packing list is root.

* Use getSourceAssetPriceCurrency
* Do not expand if no SourceAssetPrice