Abstract the support for ORDER BY and LIMIT/OFFSET SQL generation
all DB engines do not support the same syntax for these features, MS SQLServer being the bad boy we try to support in CW.
* Use two new methods of dbhelper to add LIMIT/OFFSET clauses and ORDER BY clauses
* added unit tests for sqlserver backend
* changed unittest_rql2sql to lauch the backend tests even if the driver module is not installed on the machine, so
that we can run the sqlserver tests on linux (and the mysql tests too)
* adapt msstep.py to the new interface
closes #1154756
[migration] Lesser memory consumption of cwuri fix + actually commit every 100 processed entities
Don't use the ORM as added value is poor while all intentiated object are keept
by rset cache.