Pierre-Yves David <pierre-yves.david@fb.com> [Wed, 08 Oct 2014 22:10:15 -0700] rev 22845
obsolete: store metadata as a tuple of (key, value) pairs (API)
Different formats will encode metadata in different ways. So we cannot keep the
binary blob in the object anymore. We use a tuple to ensure it is immutable and
hashable.