dht: fix sendListen loop bug

Each time a 'listen' request is sent to the network engine, we track this
request in the upper layer with a map<shared_ptr<Query>, shared_ptr<Request>>.
At some point, this map has to be maintained or totally cleared. However, it was
done wrong, hence the sendListen loop. Clearing a pair <Query, Request> has to
be done when the API user cancels a 'listen' operation.