Rework getAvailableIp

This commit reworks getAvailableIp with a "simpler" version that will
look for the first available IP address in our IP Prefix.

There is a couple of ideas behind this:

* Make the host IPs reasonably predictable and in within similar
  subnets, which should simplify ACLs for subnets
* The code is not random, but deterministic so we can have tests
* The code is a bit more understandable (no bit shift magic)

utils_test.go

@@ -0,0 +1,105 @@
+package headscale
+
+import (
+	"gopkg.in/check.v1"
+	"inet.af/netaddr"
+)
+
+func (s *Suite) TestGetAvailableIp(c *check.C) {
+	ip, err := h.getAvailableIP()
+
+	c.Assert(err, check.IsNil)
+
+	expected := netaddr.MustParseIP("10.27.0.0")
+
+	c.Assert(ip.String(), check.Equals, expected.String())
+}
+
+func (s *Suite) TestGetUsedIps(c *check.C) {
+	ip, err := h.getAvailableIP()
+	c.Assert(err, check.IsNil)
+
+	n, err := h.CreateNamespace("test_ip")
+	c.Assert(err, check.IsNil)
+
+	pak, err := h.CreatePreAuthKey(n.Name, false, false, nil)
+	c.Assert(err, check.IsNil)
+
+	_, err = h.GetMachine("test", "testmachine")
+	c.Assert(err, check.NotNil)
+
+	m := Machine{
+		ID:             0,
+		MachineKey:     "foo",
+		NodeKey:        "bar",
+		DiscoKey:       "faa",
+		Name:           "testmachine",
+		NamespaceID:    n.ID,
+		Registered:     true,
+		RegisterMethod: "authKey",
+		AuthKeyID:      uint(pak.ID),
+		IPAddress:      ip.String(),
+	}
+	h.db.Save(&m)
+
+	ips, err := h.getUsedIPs()
+
+	c.Assert(err, check.IsNil)
+
+	expected := netaddr.MustParseIP("10.27.0.0")
+
+	c.Assert(ips[0], check.Equals, expected)
+}
+
+func (s *Suite) TestGetMultiIp(c *check.C) {
+	n, err := h.CreateNamespace("test-ip-multi")
+	c.Assert(err, check.IsNil)
+
+	for i := 1; i <= 350; i++ {
+		ip, err := h.getAvailableIP()
+		c.Assert(err, check.IsNil)
+
+		pak, err := h.CreatePreAuthKey(n.Name, false, false, nil)
+		c.Assert(err, check.IsNil)
+
+		_, err = h.GetMachine("test", "testmachine")
+		c.Assert(err, check.NotNil)
+
+		m := Machine{
+			ID:             0,
+			MachineKey:     "foo",
+			NodeKey:        "bar",
+			DiscoKey:       "faa",
+			Name:           "testmachine",
+			NamespaceID:    n.ID,
+			Registered:     true,
+			RegisterMethod: "authKey",
+			AuthKeyID:      uint(pak.ID),
+			IPAddress:      ip.String(),
+		}
+		h.db.Save(&m)
+	}
+
+	ips, err := h.getUsedIPs()
+
+	c.Assert(err, check.IsNil)
+
+	c.Assert(len(ips), check.Equals, 350)
+
+	c.Assert(ips[0], check.Equals, netaddr.MustParseIP("10.27.0.0"))
+	c.Assert(ips[9], check.Equals, netaddr.MustParseIP("10.27.0.9"))
+	c.Assert(ips[300], check.Equals, netaddr.MustParseIP("10.27.1.44"))
+
+	expectedNextIP := netaddr.MustParseIP("10.27.1.94")
+	nextIP, err := h.getAvailableIP()
+	c.Assert(err, check.IsNil)
+
+	c.Assert(nextIP.String(), check.Equals, expectedNextIP.String())
+
+	// If we call get Available again, we should receive
+	// the same IP, as it has not been reserved.
+	nextIP2, err := h.getAvailableIP()
+	c.Assert(err, check.IsNil)
+
+	c.Assert(nextIP2.String(), check.Equals, expectedNextIP.String())
+}