C++ HelloWorld messaging client crash closing AMQP 1.0 connection on Windows

HelloWorld connects, send all messages, and then crashes with an access violation during connection::close(). Gordon and I tracked the issue down to a double free of TcpTransport aio objects, with the __vftable containing all 0xdddddddd pointers indicating that the object was already deleted.

Without the proposed fix, a locked 'closed' flag, the code crashes about 10% of the time. With the fix it proceeds indefinitely.