Reuse THD for new user connections

- To ensure that mallocs are marked for the correct THD, even if it's
  allocated in another thread, I added the thread_id to the THD constructor
- Added st_my_thread_var to thr_lock_info_init() to avoid a call to my_thread_var
- Moved things from THD::THD() to THD::init()
- Moved some things to THD::cleanup()
- Added THD::free_connection() and THD::reset_for_reuse()
- Added THD to CONNECT::create_thd()
- Added THD::thread_dbug_id and st_my_thread_var->dbug_id. These are needed
  to ensure that we have a constant thread_id used for debugging with a THD,
  even if it changes thread_id (=connection_id)
- Set variables.pseudo_thread_id in constructor. Removed not needed sets.

plugin/handler_socket/handlersocket/database.cpp

@@ -278,7 +278,7 @@ dbcontext::init_thread(const void *stack_bottom, volatile int& shutdown_flag)
   DBG_THR(fprintf(stderr, "HNDSOCK init thread\n"));
   {
     my_thread_init();
-    thd = new THD;
+    thd = new THD(0);
     thd->thread_stack = (char *)stack_bottom;
     DBG_THR(fprintf(stderr,
       "thread_stack = %p sizeof(THD)=%zu sizeof(mtx)=%zu "