improved sqlserver 'doModifyLimitQuery' select-from pattern

the current regex-pattern in the `doModifyLimitQuery`-function is very slow and buggy for my sqlserver queries.
i think the slowness is a result from the atomic-group inside the regex.

I tried to improve it, but since I'm not an regex-expert at all, I don't know if the 'fix' satisfies all needs.
the unit-tests returned ok.

it would be perfect if someone with some regex-knowledge would have a look at this before merging it.
the function `preg_replace` (http://www.php.net/manual/en/function.preg-replace.php) returns null on failure (like on a timeout), so I used `assertNotNull`.