Slow params generation with a lot of nested joins

When a query has many joins nested, the construction of params is very slow. This is mainly because params is called twice in Join, once for hasattr test and and second times to extend the list. As the joins are nested, this has an O(2*n!) and so it can be very slow pretty quickly.

Ref: https://discuss.tryton.org/t/performance-issue-computing-query-params/7223