Content Loader: Race condition registering readers vs. loading content

in SLING-11168 we detected a specific problem with Composum in Sling Starter 12, which only occured on Windows 10 environments (and even there not consistently in every scenario).

it seems that it may happen that Sling-Initial-Content is loaded from the bundles when the ContentReader implementations are not yet registered on the ContentReaderWhiteboard.

when this happens, the content is loaded into the repository, but files like *.json are not transformed to nodes and properties, but is stored as binary json files in the repository.