Replace the linear scan over all peers with an ipToKey map maintained
by AddPeer/RemovePeer, covering both IPv4 and IPv6 tunnel addresses.
Offline peers are intentionally no longer resolvable by IP: only active
peers can carry traffic, so IdentityForIP and the DNS disconnected-peer
filter now treat them as unknown, same as foreign IPs.
Skip the DNS answer filter for single-record responses; dropping the
only answer was always restored by the empty-answer escape hatch, so
the fast path is behavior-neutral.
3c3e8a6169