[rset] kill the rset._rqlst cache Right now it "works" for the standard, internal uses. However when we will fold ClientConnection and Connection, it will hurt, because suddenly we get more cache hits, and the following situation would become commonplace: * there is an un-annotated _rqlst given by the querier * some view (e.g. facets) requests the .syntax_tree, which takes a copy of _rqlst * the view actually expects the rql syntax tree to be annotated, but it was not, hence we crash. Related to #3837233.