Be liberal in accepting KEY_CHANGED/REQ_KEY/ANS_KEY requests.

When we got a key request for or from a node we don't know, we disconnected the
node that forwarded us that request.  However, especially in TunnelServer mode,
disconnecting does not help. We now ignore such requests, but since there is no
way of telling the original sender that the request was dropped, we now retry
sending REQ_KEY requests when we don't get an ANS_KEY back.