track b2bsum of previous version PCK

[Terkwood]

this is a breaking change to the delta-server API.

we should track the b2bsum of the PCK file for previous_version. because it will help us make sure that we're applying the patch to the intended PCK file.

we've noticed that during the export process, as the developer, it's easy to sometimes create mismatched PCK files in terms of the file contained within, for instance, the Mac OSX DMG, ... versus a PCK file exported specifically as a PCK.

tracking the b2bsum of the previous_version pck file will thus help reduce insanity, as we would know whether the diff we download actually matches its input!