Fix check timestamp and add unittest

review1087003
Fix duplicate translations

- Fix unique constraint by using -1 instead of NULL for res_id
- Clean duplicate entries in po file

The unique constraint was not working when res_id was NULL because SQL consider
all of them as different.

The migration could fail if there is duplicate translation in the database. So
this query must be run until no more records are deleted:

DELETE FROM ir_translation WHERE id IN (SELECT MAX(id) FROM ir_translation GROUP BY name, res_id, lang, type, src_md5, module HAVING COUNT(1) > 1);

issue3355
review1087004
Commit cursor in timestamp unittest

cursor must be committed between each changes otherwise NOW() returns always
the same timestamp.
Remove old test in write to update MPTT for only one record at a time