Add CHANNEL_CLOSED reason to specs

I was recently looking at stem debug logs and found

2020-03-17 21:10:58,838 stem [INFO] CIRC event had an unrecognized remote_reason (CHANNEL_CLOSED). Maybe a new addition to the control protocol? Full Event: 'CIRC 6 FAILED $0CDD60E4015EBF2C3B5D32A2B9CC8FE6C98A5C33~UnivUtah3 PURPOSE=GENERAL TIME_CREATED=2020-03-17T20:10:25.388134 REASON=DESTROYED REMOTE_REASON=CHANNEL_CLOSED'

I am not sure how the stem workflow works but I guess it's an implementation based on the spec(s) (not what is actually used/allowed in the tor code).

Thus, we should fix the spec(s) first so we can add the respective stem part later on.

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