mixclient: Use newest (fewest-PR) KEs to form alt sessions

The alternate session forming only incrementally removes PRs from the
currently considered PR set.  Even if a PR by a responsive peer is known, if
it was removed due to not passing the majority checks earlier, it will never
be used by our peers during this epoch.  With that in mind, we should only use
the most recent KEs with the lowest referenced PR counts when trying to form
an alternate session, as the additional PRs from earlier KEs will never be
reconsidered by that KE's identity.

mixing/mixclient/client.go

@@ -1908,17 +1908,17 @@ func (c *Client) alternateSession(pairing []byte, prs []*wire.MsgMixPairReq, d *
 	kes := c.mixpool.ReceiveKEsByPairing(pairing, unixEpoch)
 
 	// Sort KEs by identity first (just to group these together) followed
-	// by the total referenced PR counts in decreasing order.
-	// When ranging over KEs below, this will allow us to consider the
-	// order in which other peers created their KEs, and how they are
-	// forming their sessions.
+	// by the total referenced PR counts in increasing order (most recent
+	// KEs first).  When ranging over KEs below, this will allow us to
+	// consider the order in which other peers created their KEs, and how
+	// they are forming their sessions.
 	sort.Slice(kes, func(i, j int) bool {
 		a := kes[i]
 		b := kes[j]
 		if bytes.Compare(a.Identity[:], b.Identity[:]) == -1 {
 			return true
 		}
-		if len(a.SeenPRs) > len(b.SeenPRs) {
+		if len(a.SeenPRs) < len(b.SeenPRs) {
 			return true
 		}
 		return false
@@ -1931,7 +1931,7 @@ func (c *Client) alternateSession(pairing []byte, prs []*wire.MsgMixPairReq, d *
 		prHashByIdentity[pr.Identity] = pr.Hash()
 	}
 
-	// Only one KE per peer identity (the KE that references the most PR
+	// Only one KE per peer identity (the KE that references the least PR
 	// hashes) is used for determining session agreement.
 	type peerMsgs struct {
 		pr *wire.MsgMixPairReq