Add stall-related status variables to thread pool

It might make sense to add some stall-related status variables to the Unix implementation of the thread pool.

Maybe like:

• Threadpool_stalled_thread_groups

This could get incremented if the timer thread detects a stall. Around here:

https://github.com/MariaDB/server/blob/3503fbbebf90cb0fe63993a66dad9bf97fb74c0a/sql/threadpool_generic.cc#L735

• Threadpool_throttled_thread_creations

This could get incremented if the timer thread does not create a new worker thread when a stall is detected because the thread group was still within the throttling interval. Around here:

https://github.com/MariaDB/server/blob/3503fbbebf90cb0fe63993a66dad9bf97fb74c0a/sql/threadpool_generic.cc#L1025

• Threadpool_oversubscribed_thread_groups

This could get incremented every time a thread has to go to sleep because the thread group already has thread_pool_oversubscribe active threads. Around here:

https://github.com/MariaDB/server/blob/3503fbbebf90cb0fe63993a66dad9bf97fb74c0a/sql/threadpool_generic.cc#L1218