Fixed FederatedX 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 ha_federatedx::disconnect(), cleaner approach
is to let ha_close_connection() do it.

Part of MDEV-19515 - Improve connect speed

sql/handler.cc

@@ -2664,10 +2664,6 @@ double handler::keyread_time(uint index, uint ranges, ha_rows rows)
   return cost;
 }
 
-void **handler::ha_data(THD *thd) const
-{
-  return thd_ha_data(thd, ht);
-}
 
 THD *handler::ha_thd(void) const
 {

sql/handler.h

@@ -4475,7 +4475,6 @@ class handler :public Sql_alloc
   TABLE_SHARE* get_table_share() { return table_share; }
 protected:
   /* Service methods for use by storage engines. */
-  void **ha_data(THD *) const;
   THD *ha_thd(void) const;
 
   /**

storage/federatedx/ha_federatedx.cc

@@ -1746,10 +1746,13 @@ ha_rows ha_federatedx::records_in_range(uint inx, key_range *start_key,
 
 federatedx_txn *ha_federatedx::get_txn(THD *thd, bool no_create)
 {
-  federatedx_txn **txnp= (federatedx_txn **) ha_data(thd);
-  if (!*txnp && !no_create)
-    *txnp= new federatedx_txn();
-  return *txnp;
+  federatedx_txn *txn= (federatedx_txn *) thd_get_ha_data(thd, federatedx_hton);
+  if (!txn && !no_create)
+  {
+    txn= new federatedx_txn();
+    thd_set_ha_data(thd, federatedx_hton, txn);
+  }
+  return txn;
 }
 
 
@@ -1757,7 +1760,6 @@ int ha_federatedx::disconnect(handlerton *hton, MYSQL_THD thd)
 {
   federatedx_txn *txn= (federatedx_txn *) thd_get_ha_data(thd, hton);
   delete txn;
-  *((federatedx_txn **) thd_ha_data(thd, hton))= 0;
   return 0;
 }