message: Simplify `map` write to exclusive write access
diff: @@ -539,9 +539,9 @@ where
  DB: Database + 'db,
  Q: QueryFunction<DB>,
  {
  + // FIXME(#24) -- handle parallel case
  fn drop(&mut self) {
  - let map = self.map.upgradable_read();
  - let mut map = RwLockUpgradableReadGuard::upgrade(map);
  + let mut map = self.map.write();
  let _ = map.remove(self.key);
  }
  }
  