Use consistently named constants for relay command specifications

The original spec uses RELAY_*: https://gitweb.torproject.org/torspec.git/tree/tor-spec.txt#n1257

The onion service specs and code use RELAY_COMMAND_*: https://gitweb.torproject.org/torspec.git/tree/rend-spec.txt#n102 https://gitweb.torproject.org/torspec.git/tree/proposals/224-rend-spec-ng.txt#n252 https://gitweb.torproject.org/tor.git/tree/src/or/or.h#n562

We should probably make these consistent. Eventually.

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