Fixed EZP-20618: Multiupload freezes on the end of the first file

Jira issue: https://jira.ez.no/browse/EZP-20618

Description
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The complete object creation done by eZContentUpload is inside a DB transaction, as a result when the PersistenceCachePurger tries to clear the cache for the newly created location, the location handler is not yet able to load this location.

Tests
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Manual + unit tests