One of the bottlenecks in the current design is the individual Item
retrieval via requestItemDelivery(). The Server does a request for
the first Item, then waits for the Resource to deliver before requesting
the next Item and so on. This patch changes the signature of the
requestItemDelivery() DBus method to support passing multiple Items
(Item IDs to be specific) at once. The patch also deprecates
ResourceBase::retrieveItem() in favor of a newly introduces
ResourceBase::retrieveItems() overload (which takes Item::List and
set of part names as arguments) allowing resource implementations to
process all the requested Items in a single batch and pass them back
to the Server. This should be much faster than querying the Items one
by one.

This change does not remove the retrieveItem() method, but only
deprecates it. When Resource only implements retrieveItem() and not
the new retrieveItems() then retrieveItems() will split the ResourceScheduler
Task into many single-Item Tasks and will call retrieveItem() for
each of them.

The next step is to get rid of DBus in here and use a dedicated
command bus where the Server could request the Items via the Protocol
and would be handed the Items back through the bus.

Once all resources are ported to the new API, the old retrieveItem()
method should be removed.