Always sort WAL segments on restore point calc

Some S3 implementations, like Storj, don't return objects in order
unlike the real S3 API. It's against AWS documentation but they
have their reasons not to pre-sort them.

Since there's only one place where the order matters and we're
going to iterate all of them anyway and there's a sort
implementation for a WAL segment slice we can do just that to
add compatibility without any meaningful performance hit.

This same sorting is already done when listing snapshots.

Fixes benbjohnson#535

replica.go

@@ -1287,10 +1287,15 @@ func (r *Replica) walSegmentMap(ctx context.Context, generation string, maxIndex
 	}
 	defer itr.Close()
 
-	m := make(map[int][]int64)
+	a := []WALSegmentInfo{}
 	for itr.Next() {
-		info := itr.WALSegment()
+		a = append(a, itr.WALSegment())
+	}
 
+	sort.Sort(WALSegmentInfoSlice(a))
+
+	m := make(map[int][]int64)
+	for _, info := range a {
 		// Exit if we go past the max timestamp or index.
 		if !maxTimestamp.IsZero() && info.CreatedAt.After(maxTimestamp) {
 			break // after max timestamp, skip