Account for racecondition in deleting/closing update channel

This commit tries to address the possible raceondition  that can happen
if a client closes its connection after we have fetched it from the
syncmap before sending the message.

To try to avoid introducing new dead lock conditions, all messages sent
to updateChannel has been moved into a function, which handles the
locking (instead of calling it all over the place)

The same lock is used around the delete/close function.

app.go

@@ -58,7 +58,8 @@ type Headscale struct {
 	aclPolicy *ACLPolicy
 	aclRules  *[]tailcfg.FilterRule
 
-	clientsUpdateChannels sync.Map
+	clientsUpdateChannels     sync.Map
+	clientsUpdateChannelMutex sync.Mutex
 
 	lastStateChange sync.Map
 }