Add proper unlocking for MariaDB_FUNCTION_PLUGIN

UNINSTALL is not safe for MariaDB_FUNCTION_PLUGIN at the moment.

This is the relevant code:

    Create_func *builder=

      reinterpret_cast<Plugin_function*>(plugin_decl(plugin)->info)->

        create_func();

    plugin_unlock(thd, plugin);

    return builder;

It immediately unlocks the plugin, so UNINSTALL can happen when the plugin functions is in use.

We should implement proper unlocking for plugin functions.

• The above piece of the code should not call plugin_unlock()
• Every Item_func loaded from a plugin should:
  • decrement ref counter in destructor
  • increment ref counter in clone()