Backport client handling for END_STREAM_REASON_NOROUTE

0.2.1.x Tor clients should handle the new reason 8: NOROUTE correctly by retrying it. See the commits merged in Sebastian's misc-reason branch, plus Roger's 4c948ffd.

This is worth backporting because these errors happen not infrequently, and if the exit has a bad network configuration, clients really should retry the stream.

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