Deprecate ChecksumUtils, split its logic in their proper place

This is more about ChecksumUtils and its deprecation. This class comes from times when checksum alg was MessageDigest alg name, but since we got Checksum API (see MRESOLVER-230) and is not true. Still, this same class comes handy in some tests still today. Hence, the relevant parts should be moved to their proper places, and the class should be deprecated, to be removed in Resolver 2.1.