[peerdist] Assume that most recently discovered peer can be reused

[peerdist] Assume that most recently discovered peer can be reused

The peer discovery time has a significant impact on the overall
PeerDist download speed, since each block requires an individual
discovery attempt.  In most cases, a peer that responds for block N
will turn out to also respond for block N+1.

Assume that the most recently discovered peer (for any block) probably
has a copy of the next block to be discovered, thereby allowing the
peer download attempt to begin immediately.

In the case that this assumption is incorrect, the existing error
recovery path will allow for fallback to newly discovered peers (or to
the origin server).

Suggested-by: Andreas Hammarskjöld <junior@2PintSoftware.com>
Tested-by: Andreas Hammarskjöld <junior@2PintSoftware.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>