[bugfix] fixed the concurrency issue which caused the infamous (and usually fatal) "document id and proxy id differ" exceptions.
[bugfix] fixed the concurrency issue which caused the infamous (and usually fatal) "document id and proxy id differ" exceptions.

Previous backport of rev. 7431 was incomplete due to a recent redesign of the DocumentSet class.

svn path=/branches/eXist-stable-1.2/; revision=7446

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