Close a hosted xgress conn at once when its circuit never started - #1038

 #1036

- closes the xgress outright from edgeConnXgress.close when the circuit has not started, since
  the EOF that PeerClosed sends waits on a send buffer that only runs once the circuit starts;
  Close after CompleteAcceptFailed, Close without completing the accept, and the setup-failure
  cleanup in newChildConnection now return at once instead of after CircuitStartTimeout
- drops a conn the mux rejected as a duplicate id instead of closing it, so its teardown no
  longer removes the live conn registered under that id
- bounds the end-of-circuit send in Xgress.Close with a deadline, so a stalled router channel
  cannot hold Close
- skips the circuit start timeout warning and close for an xgress that is already closed
- tests each path through newChildConnection with no circuit start, Close against a stalled data
  plane, and the start timer on a closed xgress; each test fails without its change