Improve reporting from sf_report_leaked_memory()

Other things:
- Added DBUG_EXECUTE_IF("print_allocated_thread_memory") at end of query
  to easier find not freed memory allocated by THD
- Removed free_root() from plugin_init() that did nothing.

sql/sql_plugin.cc

@@ -1646,7 +1646,6 @@ int plugin_init(int *argc, char **argv, int flags)
         }
       }
 
-      free_root(&tmp_root, MYF(MY_MARK_BLOCKS_FREE));
       tmp.state= PLUGIN_IS_UNINITIALIZED;
       if (register_builtin(plugin, &tmp, &plugin_ptr))
         goto err_unlock;