MDEV-14505 - Threads_running becomes scalability bottleneck

Instead of updating global counter, calculate Threads_running on the fly.
All threads having command != COM_SLEEP are included.

Behaviour changes:

Previously SHOW STATUS and SHOW GLOBAL STATUS returned the same values
representing global status. Now SHOW STATUS always returns 1 indicating that
current session has 1 thread running.

Previously only threads that were executing dispatch_command() or running events
were accounted by Threads_running. Now it is rough equivalent of
SELECT COUNT(*) FROM INFORMATION_SCHEMA.PROCESSLIST WHERE state!='Sleep'

sql/sql_class.h

@@ -828,6 +828,7 @@ typedef struct system_status_var
   ulonglong table_open_cache_overflows;
   double last_query_cost;
   double cpu_time, busy_time;
+  uint32_t threads_running;
   /* Don't initialize */
   /* Memory used for thread local storage */
   int64 max_local_memory_used;