Unique Queue

Allows other modules to create unique queue items. Uniqueness is only maintained per queue and for non-claimed items. If a queue item with the same name and key exists but has already been claimed a new queue item will be created. This prevents race conditions where the new queue item may need to access data that was not available to the currently processing item.

Note: This module currently only supports unique queue items in the system queue but provides an interface that other modules may implement for additional queue back ends.

This is based on #1548286: API for handling unique queue items.