Fixed and improved locking implementations

src/pretix/base/models.py

@@ -366,7 +366,6 @@ class Event(Versionable):
         null=True, blank=True,
         verbose_name=_("Plugins"),
     )
-    locked_here = False
 
     class Meta:
         verbose_name = _("Event")
@@ -1822,6 +1821,10 @@ class OrganizerSetting(Versionable):
 class EventLock(models.Model):
     event = models.CharField(max_length=36, primary_key=True)
     date = models.DateTimeField(auto_now=True)
+    token = models.UUIDField(default=uuid.uuid4)
 
     class LockTimeoutException(Exception):
         pass
+
+    class LockReleaseException(Exception):
+        pass