We only set [epAddr]s in the [peerMap] when wireguard-go tells us who
they belong to. A node key can only have a single [epAddr] in the
[peerMap].
We also clear an [epAddr] when wireguard-go tells us our
mapping assumption between [epAddr] and peer is wrong (outdated).
Signed-off-by: Jordan Whited <jordan@tailscale.com>