Resolve race condition in saveDocuments

[lnrd96]

User story

1. As a user I want to upload documents to either add new items or to update existing ones.
2. I need to persist the provided information even if a colleague is simultaneously changing the tag field of an item that I update by uploading the document. The same holds for multiple saveDocuments requests.
3. So that both changes are persisted.

Acceptance criteria

Fix all race conditions in the function.
Simultaneously incoming requests get processed subsequently.

Definition of done (DoD)

Feature has been fully implemented
No known errors
All acceptance criteria were met
Test passes
Peer code review passed