#915 Session close method duplicated in c2s, s2s, bosh, websocket
Each of mod_c2s, mod_s2s, mod_bosh and mod_websocket have their own session.close definition, which each include the sending of stream errors. This should be unified in util.session. The stream error handling should probably be broken out as well. There should be suitable extension places for transport specific bits (compare stream opening).