tuned tracing;

replaced Theory.self_ref by Theory.check_thy, which now produces a checked ref; thread-safeness: when creating certified items, perform Theory.check_thy *last*;

certify: no check_thy here;

improved check_thy: produce a checked theory_ref (thread-safe version); removed obsolete self_ref (cf. check_thy); thread-safeness: when creating certified items, perform Theory.check_thy *last*; eq_thy: no check here; marked some critical sections;