Memoizer hangs in some `class` initializations

It can happen that the `Memoizer` blocks and never finishes on https://github.com/commercetools/sphere-scala-libs/blob/main/util/src/main/scala/Memoizer.scala#L20 while creating ´JSON´ instances. It looks to me like a concurrency issue, but I don't know how to reproduce the issue in a general way (can't share my code).

Read util/src/main/scala/Memoizer.scala around line 20 and inspect how JSON instances are created during class initialization. Establish a reproducible hang or regression test for the reported blocking behavior, then use it to verify the Memoizer no longer hangs.

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