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/handler_socket/handlersocket/database.cpp

@@ -308,11 +308,9 @@ dbcontext::init_thread(const void *stack_bottom, volatile int& shutdown_flag)
     DBG_THR(fprintf(stderr, "HNDSOCK x0 %p\n", thd));
   }
   {
-    pthread_mutex_lock(&LOCK_thread_count);
-    thd->thread_id = thread_id++;
-    threads.append(thd);
-    ++thread_count;
-    pthread_mutex_unlock(&LOCK_thread_count);
+    thd->thread_id = next_thread_id();
+    thread_safe_increment32(&thread_count);
+    add_to_active_threads(thd);
   }
 
   DBG_THR(fprintf(stderr, "HNDSOCK init thread wsts\n"));