cmd/natc: add optional consensus backend

Enable nat connector to be run on a cluster of machines for high
availability.

Updates #14667

Signed-off-by: Fran Bull <fran@tailscale.com>

cmd/natc/ippool/ippool_test.go

@@ -8,6 +8,7 @@ import (
 	"fmt"
 	"net/netip"
 	"testing"
+	"time"
 
 	"go4.org/netipx"
 	"tailscale.com/tailcfg"
@@ -19,7 +20,7 @@ func TestIPPoolExhaustion(t *testing.T) {
 	var ipsb netipx.IPSetBuilder
 	ipsb.AddPrefix(smallPrefix)
 	addrPool := must.Get(ipsb.IPSet())
-	pool := IPPool{IPSet: addrPool}
+	pool := SingleMachineIPPool{IPSet: addrPool}
 
 	assignedIPs := make(map[netip.Addr]string)
 
@@ -68,7 +69,7 @@ func TestIPPool(t *testing.T) {
 	var ipsb netipx.IPSetBuilder
 	ipsb.AddPrefix(netip.MustParsePrefix("100.64.1.0/24"))
 	addrPool := must.Get(ipsb.IPSet())
-	pool := IPPool{
+	pool := SingleMachineIPPool{
 		IPSet: addrPool,
 	}
 	from := tailcfg.NodeID(12345)
@@ -89,7 +90,7 @@ func TestIPPool(t *testing.T) {
 		t.Errorf("IPv4 address %s not in range %s", addr, addrPool)
 	}
 
-	domain, ok := pool.DomainForIP(from, addr)
+	domain, ok := pool.DomainForIP(from, addr, time.Now())
 	if !ok {
 		t.Errorf("domainForIP(%s) not found", addr)
 	} else if domain != "example.com" {