ReverifyPiece() is not currently hooked up to anything, but is planned
to take the place of audit.(*Verifier).Reverify().
ReverifyPiece() works by downloading one piece in its entirety, rather
than pulling an entire stripe across many nodes.
Change-Id: Ie2c680f4d3c3b65273a72466a3f9f55c115b0311
