Fixed Mroonga to follow THD ha_data protocol

Use thd_get_ha_data()/thd_set_ha_data() which protect against plugin
removal until it has THD ha_data.

Do not reset THD ha_data in mrn_close_connection(), cleaner approach
is to let ha_close_connection() do it.

Part of MDEV-19515 - Improve connect speed

storage/mroonga/ha_mroonga.cpp

@@ -1377,11 +1377,10 @@ static void mrn_drop_database(handlerton *hton, char *path)
 static int mrn_close_connection(handlerton *hton, THD *thd)
 {
   MRN_DBUG_ENTER_FUNCTION();
-  void *p = *thd_ha_data(thd, mrn_hton_ptr);
+  void *p = thd_get_ha_data(thd, mrn_hton_ptr);
   if (p) {
     mrn_clear_slot_data(thd);
     free(p);
-    *thd_ha_data(thd, mrn_hton_ptr) = (void *) NULL;
     {
       mrn::Lock lock(&mrn_allocated_thds_mutex);
       my_hash_delete(&mrn_allocated_thds, (uchar*) thd);
@@ -2040,7 +2039,7 @@ static int mrn_deinit(void *p)
       mrn_clear_slot_data(tmp_thd);
       void *slot_ptr = mrn_get_slot_data(tmp_thd, false);
       if (slot_ptr) free(slot_ptr);
-      *thd_ha_data(tmp_thd, mrn_hton_ptr) = (void *) NULL;
+      thd_set_ha_data(tmp_thd, mrn_hton_ptr, 0);
       my_hash_delete(&mrn_allocated_thds, (uchar *) tmp_thd);
     }
   }

storage/mroonga/mrn_table.cpp

@@ -1140,7 +1140,7 @@ st_mrn_slot_data *mrn_get_slot_data(THD *thd, bool can_create)
 {
   MRN_DBUG_ENTER_FUNCTION();
   st_mrn_slot_data *slot_data =
-    (st_mrn_slot_data*) *thd_ha_data(thd, mrn_hton_ptr);
+    (st_mrn_slot_data*) thd_get_ha_data(thd, mrn_hton_ptr);
   if (slot_data == NULL) {
     slot_data = (st_mrn_slot_data*) malloc(sizeof(st_mrn_slot_data));
     slot_data->last_insert_record_id = GRN_ID_NIL;
@@ -1149,7 +1149,7 @@ st_mrn_slot_data *mrn_get_slot_data(THD *thd, bool can_create)
     slot_data->disable_keys_create_info = NULL;
     slot_data->alter_connect_string = NULL;
     slot_data->alter_comment = NULL;
-    *thd_ha_data(thd, mrn_hton_ptr) = (void *) slot_data;
+    thd_set_ha_data(thd, mrn_hton_ptr, slot_data);
     {
       mrn::Lock lock(&mrn_allocated_thds_mutex);
       if (my_hash_insert(&mrn_allocated_thds, (uchar*) thd))