Adds the ability to compute a "multihash" (see the Glance spec
for what this is exactly). To maintain backward compatability,
a new store_add_to_backend_with_multihash function is added.
Backward compatability for each store's add() method is achieved
by a back_compat_add wrapper.
Co-Authored-by: Scott McClymont <scott.mcclymont@verizonwireless.com>
Co-Authored-by: Brian Rosmaita <rosmaita.fossdev@gmail.com>
Change-Id: I063d0900b7dc7e0d94dfb685971eb9b17ed67c7b
Partially-implements: blueprint multihash
ba9808cebb