45628143: Store fixity data when a datastream is created or modified

Uses org.modeshape.jcr.api.Binary for retrieving SHA-1 digests rather than calculating anew.
Also see: https://docs.jboss.org/author/display/MODE/Binary+values

I think this means we should give up the pretense that we support digest algorithms other than SHA-1,
which I'm ok with.

https://www.pivotaltracker.com/story/show/45628143