Compute the recursive parameter depending on the query flag for recursivity,
enabling it only if the query recursive flag is NAUTILUS_QUERY_RECURSIVE_ALWAYS,
while should be disabled otherwise.

At this point the "recursive" property that was set only for this search engine
doesn't make any sense anymore and we can safely drop it, together with the
calls that were done at search-engine level to handle this special case.

We move now the responsibility to to the engine itself, more than to the model.