Update QGIS WFS Server to use the new vector API

This update needs to be tested. It should primarily test the
PROPERTYNAME parameter for GetFeature request, and test TRANSACTION.
This update it's just for compatibility. It should be good to update the
code to use all the new vector API capability.
Merge pull request #392 from rldhont/new_vector_api

Update QGIS WFS Server to use the new vector API