This adds the capability to the segment-verify tool of checking all
pieces of every indicated segment.
Pieces which could not be accessed (i.e. we couldn't get a single
byte from them) are recorded in a csv file.
I haven't been able to test this in any very meaningful way, yet, but I
am comforted by the fact that the worst things it could possibly do are
(a) download pieces too many times, and (b) miss downloading some
pieces.
Change-Id: I3aba30921572c974993363eb36d0fd5b3ae97907
727136141a