Bookshelves don't update their infotext properly

[Montandalar]

The local function update_bookshelf creates the metadata formspec attached to a bookshelf and sets its infotext according to the number of books inside.

However, in the current implementation this function is only called in the node's on_construct, and never again afterwards. If you place books inside the bookshelf and take them out, the infotext will never change. The inventory handler callbacks are set to log actions to the server log, but they should also update the infotext.