Let the propertiesupdate section support dexterity.

Dexterity objects do not provide archetypes IBaseObject, so checking for
the _setProperty method is sufficient.

Also: always yield the item when continuing.
Improve performance by loading the items in a thread.

Loading each item with HTTP is very time consuming.
By loading the items in a thread the main migration thread does no longer need to
wait while loading each item.

The queue does only load a maximum amount of items (default: 10) so that the RAM
does not get filled up.
Merge branch 'jone-performance'
Merge branch 'jone-properties-dexterity'