GH-2795: Fix race condition in datatype registration

[Ostrzyciel]

GitHub issue resolved #2795

Pull request Description:

Fixed a race condition in user-defined datatype registration. This change only touches the user-friendly getSafeTypeByName and does not change anything from the user's perspective (other than fixing the race condition). The registerDatatype method is left as-is, to allow for overriding datatype registrations if someone really wants to.

I've added some notes to registerDatatype and unregisterDatatype that it they have good "shoot-yourself-in-the-foot-with-a-bazooka" potential.

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• Tests are included – not sure how to test it... do we need tests here?
• Documentation change and updates are provided for the Apache Jena website
• Commits have been squashed to remove intermediate development commit messages.
• Key commit messages start with the issue number (GH-xxxx)

By submitting this pull request, I acknowledge that I am making a contribution to the Apache Software Foundation under the terms and conditions of the Contributor's Agreement.

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See the Apache Jena "Contributing" guide.

[arne-bdt]

I don´t think, this is the way to solve the issue.
--> discussion in #2795 (comment)

[Ostrzyciel]

Updated the PR with a different approach as per the discussion in the issue #2795