Add item loading/unloading with a timeout

[rakshasa]

Currently one needs to use async retrieve with wait_for_loaded to attempt to load an item with a timeout.

New methods should be added to request an item to be loaded/unloaded, with timeouts. In addition to the timeouts these should also check if there is already attempts to load an item, and if so not duplicate the db access and event creations.

[rakshasa]

This has been added already.