TS-2578: Close the client connection when you close TransformTerminus

If a transform has sent any content downstream before it decides
to call TSVConnAbort(output_vc) or TSVConnClose(output_vc) then the
client connection doesn't get closed. If TransformTerminus::handle_event()
sends VC_EVENT_ERROR or VC_EVENT_EOS events then the client connection
does get closed, but it only sends these if m_called_user is false.
After any content has been sent downstream, m_called_user is true.

GitHub: This closes #42