Fix endless loop in search_api_index_specific_items_delayed()

Problem/Motivation

If search_api_index_specific_items_delayed() is called it registers _search_api_index_queued_items() as a drupal shutdown callback.
And it does this as long as the static variable $queue is empty.
The problem is if I call search_api_index_specific_items_delayed() to check if there's a queue, I omit the index and ids parameter. This means an empty queue is created and the shutdown function is registered.
This leads to an endless loop since _search_api_index_queued_items() also calls search_api_index_specific_items_delayed() to fetch the queue - since the queue is empty another shutdown callback is registered. Because the whole thing works with a reference the loop over the shutdown function gets immediately a new callback, which will add also a new callback and so on.

Proposed resolution

Add another static variable, that indicates whether the shutdown callback is registered.

Remaining tasks

Review

User interface changes

none

API changes

none