wgengine/magicsock: add new discoInfo type for DiscoKey state, move some fields

As more prep for removing the false assumption that you're able to
map from DiscoKey to a single peer, move the lastPingFrom and lastPingTime
fields from the endpoint type to a new discoInfo type, effectively upgrading
the old sharedDiscoKey map (which only held a *[32]byte nacl precomputed key
as its value) to discoInfo which then includes that naclbox key.

Then start plumbing it into handlePing in prep for removing the need
for handlePing to take an endpoint parameter.

Updates #3088

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

wgengine/magicsock/magicsock_test.go

@@ -1158,7 +1158,7 @@ func TestDiscoMessage(t *testing.T) {
 	pkt = append(pkt, nonce[:]...)
 
 	pkt = box.Seal(pkt, []byte(payload), &nonce, c.discoPrivate.Public().B32(), peer1Priv.B32())
-	got := c.handleDiscoMessage(pkt, netaddr.IPPort{}, key.Public{})
+	got := c.handleDiscoMessage(pkt, netaddr.IPPort{}, tailcfg.NodeKey{})
 	if !got {
 		t.Error("failed to open it")
 	}