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.

handshake_manager_test.go

@@ -103,6 +103,56 @@ func Test_NewHandshakeManagerVpnIP(t *testing.T) {
 	}
 }
 
+func Test_NewHandshakeManagerTrigger(t *testing.T) {
+	_, tuncidr, _ := net.ParseCIDR("172.1.1.1/24")
+	_, vpncidr, _ := net.ParseCIDR("172.1.1.1/24")
+	_, localrange, _ := net.ParseCIDR("10.1.1.1/24")
+	ip := ip2int(net.ParseIP("172.1.1.2"))
+	preferredRanges := []*net.IPNet{localrange}
+	mw := &mockEncWriter{}
+	mainHM := NewHostMap("test", vpncidr, preferredRanges)
+	lh := &LightHouse{}
+
+	blah := NewHandshakeManager(tuncidr, preferredRanges, mainHM, lh, &udpConn{}, defaultHandshakeConfig)
+
+	now := time.Now()
+	blah.NextOutboundHandshakeTimerTick(now, mw)
+
+	assert.Equal(t, 0, testCountTimerWheelEntries(blah.OutboundHandshakeTimer))
+
+	blah.AddVpnIP(ip)
+
+	assert.Equal(t, 1, testCountTimerWheelEntries(blah.OutboundHandshakeTimer))
+
+	// Trigger the same method the channel will
+	blah.handleOutbound(ip, mw, true)
+
+	// Make sure the trigger doesn't schedule another timer entry
+	assert.Equal(t, 1, testCountTimerWheelEntries(blah.OutboundHandshakeTimer))
+	hi := blah.pendingHostMap.Hosts[ip]
+	assert.Nil(t, hi.remote)
+
+	lh.addrMap = map[uint32][]udpAddr{
+		ip: {*NewUDPAddrFromString("10.1.1.1:4242")},
+	}
+
+	// This should trigger the hostmap to populate the hostinfo
+	blah.handleOutbound(ip, mw, true)
+	assert.NotNil(t, hi.remote)
+	assert.Equal(t, 1, testCountTimerWheelEntries(blah.OutboundHandshakeTimer))
+}
+
+func testCountTimerWheelEntries(tw *SystemTimerWheel) (c int) {
+	for _, i := range tw.wheel {
+		n := i.Head
+		for n != nil {
+			c++
+			n = n.Next
+		}
+	}
+	return c
+}
+
 func Test_NewHandshakeManagerVpnIPcleanup(t *testing.T) {
 	_, tuncidr, _ := net.ParseCIDR("172.1.1.1/24")
 	_, vpncidr, _ := net.ParseCIDR("172.1.1.1/24")