Fixes to `MsgHelpers`:
- Add missing lock in `setPkgMsgLevel` to ensure `setLevel` is called
  while holding the lock.
- Mark the static `MsgStream` map as thread-safe (protected by mutex).
- Remove the caching of the `MessageSvc` and rely on
  `Athena::getMessageSvc` (the latter does not implement caching either,
  but if really needed it should be done there).