explicitly destroying io_serv at the end of shutdown

[wanderingbort]

when the io_serv is stopped it may still own std::functions/lambdas that have captured data in them. If this captured data relies on another system like a logging system, for instance, waiting for destructors to be called by the atexit call may result in them accessing a destroyed system. By explicitly resetting the io_serve after shutdown we in turn release any resources it has while the systems are still alive and well