Revert THD::THD(skip_global_sys_var_lock) argument

Originally introduced by e972125f1 to avoid harmless wait for
LOCK_global_system_variables in a newly created thread, which creation was
initiated by system variable update.

At the same time it opens dangerous hole, when system variable update
thread already released LOCK_global_system_variables and ack_receiver
thread haven't yet completed new THD construction. In this case THD
constructor goes completely unprotected.

Since ack_receiver.stop() waits for the thread to go down, we have to
temporarily release LOCK_global_system_variables so that it doesn't
deadlock with ack_receiver.run(). Unfortunately it breaks atomicity
of rpl_semi_sync_master_enabled updates and makes them not serialized.

LOCK_rpl_semi_sync_master_enabled was introduced to workaround the above.
TODO: move ack_receiver start/stop into repl_semisync_master
enable_master/disable_master under LOCK_binlog protection?

Part of MDEV-14984 - regression in connect performance

sql/sql_class.h

@@ -3263,17 +3263,12 @@ public:
   /**
     @param id                thread identifier
     @param is_wsrep_applier  thread type
-    @param skip_lock         instruct whether @c LOCK_global_system_variables
-                             is already locked, to not acquire it then.
   */
-  THD(my_thread_id id, bool is_wsrep_applier= false, bool skip_lock= false);
+  THD(my_thread_id id, bool is_wsrep_applier= false);
 
   ~THD();
-  /**
-    @param skip_lock         instruct whether @c LOCK_global_system_variables
-                             is already locked, to not acquire it then.
-  */
-  void init(bool skip_lock= false);
+
+  void init();
   /*
     Initialize memory roots necessary for query processing and (!)
     pre-allocate memory for it. We can't do that in THD constructor because