Master can fail to detect HTTP framework disconnection if it disconnects very fast

What we've observed is that if the framework disconnects before the master actor processes the initial subscribe request, the master would set up an exited callback that never gets triggered.

It looks like it's because when the socket closes and libprocess terminates the HttpProxy for this socket, the pipe reader for this proxy is not set.

Later when the master sets up the callback, it would be a noop in this regard.