Improve concurrency of ContentRepositoryImpl

ContentRepositoryImpl has a sharable MessageDigest instance, access to which is (incompletely) controlled via synchronization. That limits the number of concurrent calls, e.g. for uploading different repository content.

The sharing is a nice to have thing, but it's not invalid to have more than 1 digest active at once, so do a bit of bookkeeping to use the shared instance if available and create a one-off instance otherwise.

Also fix the partially missing control of the shared digest object.