Move copying of global variables into thd.variables into plugin_thdvar_init().
  plugin_thdvar_init() may be called multiple times when THD::change_user() is called.
  This fixes a plugin ref-count leak when running the test mysql_client_test


sql/sql_class.cc:
  wl2936
    Initialize thd.variables to zero in constructor.
    Move copying of global variables into thd.variables into plugin_thdvar_init()
sql/sql_plugin.cc:
  wl2936
    Move copying of global variables into thd.variables into plugin_thdvar_init().
    plugin_thdvar_init() may be called multiple times when THD::change_user() is called.
    This fixes a plugin ref-count leak when running the test mysql_client_test