Update low-level i'faces to reflect wireformats.

This commit fixes a critique of the old storage API design, whereby
the input parameters were always as raw bytes and never Protocol
Buffer messages that encapsulated the data, meaning every place a
read or mutation was conducted needed to manually perform said
translations on its own.  This is taxing.

Change-Id: I4786938d0d207cefb7782bd2bd96a517eead186f
Merge "Update low-level i'faces to reflect wireformats."