MDEV-6150 Speed up connection speed by moving creation of THD to new thread

Creating a CONNECT object on client connect and pass this to the working thread which creates the THD.
Split LOCK_thread_count to different mutexes
Added LOCK_thread_start to syncronize threads
Moved most usage of LOCK_thread_count to dedicated functions
Use next_thread_id() instead of thread_id++

Other things:
- Thread id now starts from 1 instead of 2
- Added cast for thread_id as thread id is now of type my_thread_id
- Made THD->host const (To ensure it's not changed)
- Removed some DBUG_PRINT() about entering/exiting mutex as these was already logged by mutex code
- Fixed that aborted_connects and connection_errors_internal are counted in all cases
- Don't take locks for current_linfo when we set it (not needed as it was 0 before)

plugin/feedback/sender_thread.cc

@@ -280,9 +280,7 @@ pthread_handler_t background_thread(void *arg __attribute__((unused)))
   if (my_thread_init())
     return 0;
 
-  mysql_mutex_lock(&LOCK_thread_count);
-  thd_thread_id= thread_id++;
-  mysql_mutex_unlock(&LOCK_thread_count);
+  thd_thread_id= next_thread_id();
 
   if (slept_ok(startup_interval))
   {