trigger handshakes when lighthouse reply arrives (#246)

Currently, we wait until the next timer tick to act on the lighthouse's
reply to our HostQuery. This means we can easily add hundreds of
milliseconds of unnecessary delay to the handshake. To fix this, we
can introduce a channel to trigger an outbound handshake without waiting
for the next timer tick.

A few samples of cold ping time between two hosts that require a
lighthouse lookup:

    before (v1.2.0):

    time=156 ms
    time=252 ms
    time=12.6 ms
    time=301 ms
    time=352 ms
    time=49.4 ms
    time=150 ms
    time=13.5 ms
    time=8.24 ms
    time=161 ms
    time=355 ms

    after:

    time=3.53 ms
    time=3.14 ms
    time=3.08 ms
    time=3.92 ms
    time=7.78 ms
    time=3.59 ms
    time=3.07 ms
    time=3.22 ms
    time=3.12 ms
    time=3.08 ms
    time=8.04 ms

I recommend reviewing this PR by looking at each commit individually, as
some refactoring was required that makes the diff a bit confusing when
combined together.

lighthouse.go

@@ -30,6 +30,9 @@ type LightHouse struct {
 	// filters local addresses that we advertise to lighthouses
 	localAllowList *AllowList
 
+	// used to trigger the HandshakeManager when we receive HostQueryReply
+	handshakeTrigger chan<- uint32
+
 	// staticList exists to avoid having a bool in each addrMap entry
 	// since static should be rare
 	staticList  map[uint32]struct{}
@@ -358,6 +361,11 @@ func (lh *LightHouse) HandleRequest(rAddr *udpAddr, vpnIp uint32, p []byte, c *c
 			ans := NewUDPAddr(a.Ip, uint16(a.Port))
 			lh.AddRemote(n.Details.VpnIp, ans, false)
 		}
+		// Non-blocking attempt to trigger, skip if it would block
+		select {
+		case lh.handshakeTrigger <- n.Details.VpnIp:
+		default:
+		}
 
 	case NebulaMeta_HostUpdateNotification:
 		//Simple check that the host sent this not someone else