Maak team updates synchronized

src/main/java/puzzeltocht/domain/Event.java

@@ -28,15 +28,15 @@ public class Event {
         return description;
     }
 
-    public void addTeam(Team t) {
+    public synchronized void addTeam(Team t) {
         teams.put(t.getId(), t);
     }
 
-    public Collection<Team> getTeams() {
+    public synchronized Collection<Team> getTeams() {
         return Collections.unmodifiableCollection(teams.values());
     }
 
-    public Team getTeam(UUID id) {
+    public synchronized Team getTeam(UUID id) {
         return teams.get(id);
     }