Add MISSING_STORED_FIXITY state for FixityResult, allowing fixity checks to happen on e.g. projected nodes that may or may not have our custom fixity properties
only mix in fedora:datastream and fedora:object properties if we've just created the object; if it already exists, just roll it with it.
Update DatastreamService#getFixity to support InMemory and External
Binary values

Previously, getFixity only supported binary values that were persisted
into a CacheStore, ignoring projected and in-memory values. This patch
adds support for InMemory and External values by running their
InputStream through our fixity checks (and assumes these values can't be
repaired, as some configurations of cache stores can).