Other: Fix race condition when changing address mode

When changing address mode, we would close all a user's update channels
and create them from scratch. This involved setting user.updateCh to a
new value. However, it was possible for other goroutines to read from
user.updateCh during this time. I replaced it with a call to
user.updateCh.Clear(), which is threadsafe.

internal/safe/map.go

@@ -262,6 +262,14 @@ func (m *Map[Key, Val]) ValuesErr(fn func(vals []Val) error) error {
 	return err
 }
 
+func (m *Map[Key, Val]) Clear() {
+	m.lock.Lock()
+	defer m.lock.Unlock()
+
+	m.data = make(map[Key]Val)
+	m.order = nil
+}
+
 func (m *Map[Key, Val]) MapErr(fn func(map[Key]Val) error) error {
 	m.lock.RLock()
 	defer m.lock.RUnlock()