moduleapi: Use multitable add/remove instead of set multitable:set() does not clear intermediate keys when setting the last one to nil. This meant that the event_handlers multitable for every module was not properly cleaning up the object and event name from the table when calling :unhook_object_event(). This combined badly with e.g. mod_bookmarks, which uses this extensively to add/remove hooks dynamically for mod_pep services. The multitable kept a reference to every mod_pep service object ever created, causing a memory leak. The multitable:remove() function clears intermediate keys when they are empty, so we've switched to using that now.