Faster implementation for circuit_get_by_rend_token_and_purpose()
According to https://lists.torproject.org/pipermail/tor-relays/2013-September/002715.html , https://lists.torproject.org/pipermail/tor-relays/2013-September/002715.html can be 6% of a relay's runtime.
This is another function that does a linear search when a hashtable lookup would be more appropriate.
We'll need to be a little careful, since there's nothing preventing collisions here: An intro circuit and a rendezvous circuit can have the same "token" pretty easily.
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