fix: return valid AuthUrl in followup request on expired reg id

- tailscale client gets a new AuthUrl and sets entry in the regcache
- regcache entry expires
- client doesn't know about that
- client always polls followup request а gets error

When user clicks "Login" in the app (after cache expiry), they visit
invalid URL and get "node not found in registration cache". Some clients
on Windows for e.g. can't get a new AuthUrl without restart the app.

To fix that we can issue a new reg id and return user a new valid
AuthUrl.

RegisterNode is refactored to be created with NewRegisterNode() to
autocreate channel and other stuff.

hscontrol/types/common.go

@@ -7,6 +7,7 @@ import (
 	"errors"
 	"fmt"
 	"runtime"
+	"sync/atomic"
 	"time"
 
 	"github.com/juanfont/headscale/hscontrol/util"
@@ -186,6 +187,28 @@ func (r RegistrationID) String() string {
 type RegisterNode struct {
 	Node       Node
 	Registered chan *Node
+	closed     *atomic.Bool
+}
+
+func NewRegisterNode(node Node) RegisterNode {
+	return RegisterNode{
+		Node:       node,
+		Registered: make(chan *Node),
+		closed:     &atomic.Bool{},
+	}
+}
+
+func (rn *RegisterNode) SendAndClose(node *Node) {
+	if rn.closed.Swap(true) {
+		return
+	}
+
+	select {
+	case rn.Registered <- node:
+	default:
+	}
+
+	close(rn.Registered)
 }
 
 // DefaultBatcherWorkers returns the default number of batcher workers.