Replace the timestamp based state system

This commit replaces the timestamp based state system with a new
one that has update channels directly to the connected nodes. It
will send an update to all listening clients via the polling
mechanism.

It introduces a new package notifier, which has a concurrency safe
manager for all our channels to the connected nodes.

Signed-off-by: Kristoffer Dalby <kristoffer@tailscale.com>

hscontrol/notifier/notifier.go

@@ -0,0 +1,55 @@
+package notifier
+
+import (
+	"sync"
+
+	"github.com/juanfont/headscale/hscontrol/util"
+)
+
+type Notifier struct {
+	l     sync.RWMutex
+	nodes map[string]chan<- struct{}
+}
+
+func NewNotifier() *Notifier {
+	return &Notifier{}
+}
+
+func (n *Notifier) AddNode(machineKey string, c chan<- struct{}) {
+	n.l.Lock()
+	defer n.l.Unlock()
+
+	if n.nodes == nil {
+		n.nodes = make(map[string]chan<- struct{})
+	}
+
+	n.nodes[machineKey] = c
+}
+
+func (n *Notifier) RemoveNode(machineKey string) {
+	n.l.Lock()
+	defer n.l.Unlock()
+
+	if n.nodes == nil {
+		return
+	}
+
+	delete(n.nodes, machineKey)
+}
+
+func (n *Notifier) NotifyAll() {
+	n.NotifyWithIgnore()
+}
+
+func (n *Notifier) NotifyWithIgnore(ignore ...string) {
+	n.l.RLock()
+	defer n.l.RUnlock()
+
+	for key, c := range n.nodes {
+		if util.IsStringInSlice(ignore, key) {
+			continue
+		}
+
+		c <- struct{}{}
+	}
+}