CQL3 SelectStatement should not share slice query filters amongst ReadCommand

In 1.2, SliceQueryFilter is stateful and should thus not be shared but SelectStatement is doing some sharing in the case of an IN query. This is the reason why cql_test:TestCQL.limit_multiget_test in the dtests is failing intermittently.

Let's be clear that the fact that SliceQueryFilter is stateful is ugly, but that is a concession made for performance until we can refactor all this more cleanly (which still being efficient). Such refactor being, as far as I can tell, far from trivial.