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)

sql/sql_list.h

@@ -650,6 +650,10 @@ struct ilink
     if (next) next->prev=prev;
     prev=0 ; next=0;
   }
+  inline void assert_if_linked()
+  {
+    DBUG_ASSERT(prev != 0 && next != 0);
+  }
   virtual ~ilink() { unlink(); }		/*lint -e1740 */
 };