text: Title: [BUG-REPORT] writer_threads is ignored when exporting to hdf5
  Body: Thank you for reaching out and helping us improve Vaex!
  
  Before you submit a new Issue, please read through the [documentation](https://docs.vaex.io/en/latest/). Also, make sure you search through the Open and Closed Issues - your problem may already be discussed or addressed.
  
  **Description**
  Please provide a clear and concise description of the problem. This should contain all the steps needed to reproduce the problem. A minimal code example that exposes the problem is very appreciated.
  
  **Software information**
   - Vaex version (`import vaex; vaex.__version__)`: 4.3.0
   - Vaex was installed via: pip 
   - OS: ubuntu 20.04
  
  **Additional information**
  We're offering this option in API, but writer_threads is not in use
  