1. Use a intrinsic sync block instead of specific re-entrant locks using try and finally.
2. Use linked list instead of array deque to allow inserting null locks. This prevents wait forevers when we have exception in async queries
3. Make sure we don't fire queries immediately on construction if there are already too many async queries piled up.
4. Reduce the max async query limit to 100. This can be made configurable through metadata in the future