tpf-udp-proxy: expire unanswered token messages

[reduzent]

tpf-udp-proxy expects to receive the same token message from two clients for it to establish a link between them. However, it doesn't handle cases well where a token message is dropped. This can lead to situations where clients try to re-establish the connection by sending the token message again. If there is still an unanswered token message available, the first incoming completes the match and the second one waits for an answer. If clients re-use the same token (which they shouldn't do and which is fixed now in tpf-client), this might prevent them from establishing an audio connection.

Unanswered token message should be discarded after a certain time since both clients usually send their matching token messages within a very short period of time under normal network conditions.