cache without overwrite previous conversion when processed at the same time.
This test show that conversion cache design is not scallable.

All conversions are stored in a unique dictionary per Document.
This dictionary can not be modified by parallel threads because 
multiple copies of this dictionary are distributed among threads.
When The last thread finish, it take precedence and erase previous
copies of that dictionary.



git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@34817 20353a03-c40f-0410-a6d1-a30d3c3de9de