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

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