Parallel Query: Study how to create worker threads

What is the way to create another worker thread from the SQL layer?

• Initially we should use a server variable that will control how many workers to create
• We should request workers from the thread pool (right?)
  • And return them to the thread pool when they are done.

Code a basic function that will

• request N workers
• Let these workers do something
• Then return control to the main thread.
• The main thread should wait for workers to finish (and start processing their result as soon as they are done).

Followups to this task:

• workers should be intialized to use MariaDB internal environment (mysys, etc)

TODO - first things

• Study the threadpool code to answer the above questions: (AI may also help with this)
  • How to get a worker thread
  • How to return a thread

• How does one wait for multiple worker threads at the same time and handle them as soon as they terminate, provided they can do it in an arbitrary order?