Import time improvment

Instead of making as many queries as you ps_feature entries (for each
product), it is possible to make it in one query.

And we test if the user has the MySQL rights to create temporary tables.
cleanPositions query update

We now use an unique query, with no need to use temporary table.

Thanks to @sylwit for the tip !
Merge pull request #299 from samy-r/dev-import-time

Import time improvment