Halt! - implementing stopping the connection

`m_transport` might be set to null by [another thread](https://github.com/aspnet/SignalR-Client-Cpp/blob/abort/src/signalrclient/connection_impl.cpp#L124) even though you just null checked. Reordering the call to `m_start_completed_event.set()` to be after you set `m_transport` to null might be enough to fix this.