Default to fast-forward cursor for QDjangoQuery with MSSqlServer

This is a significant performance improvement for MSSqlServer connections
using the QODBC driver. Fast-forward cursors are automatically converted
to static cursors if they meet a number of criteria (most importantly
being that the statement is not a read-only operation), so defaulting
to fast-forward gives the best of all worlds.

Interesting reading: http://technet.microsoft.com/en-us/library/aa172573(v=sql.80).aspx,
in particular the section on "Implicit Conversion of Fast Forward-only Cursors"