support for version 2 of block image diffs

In version 2 of block image diffs, we support a new command to load
data from the image and store it in the "stash table" and then
subsequently use entries in the stash table to fill in missing bits of
source data we're not allowed to read when doing move/bsdiff/imgdiff
commands.

This leads to smaller update packages because we can break cycles in
the ordering of how pieces are updated by storing data away and using
it later, rather than not using the data as input to the patch system
at all.  This comes at the cost of the RAM or scratch disk needed to
store the data.

The implementation is backwards compatible; it can still handle the
existing version 1 of the transfer file format.

Change-Id: I4559bfd76d5403859637aeac832f3a5e9e13b63a

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