ResponsiveDownloadStrategy will readahead blocks

Test peers don't play well with short "up request" buffers, so we need to fill
these with the most likely blocks to achieve maximum download speeds.

client.go

@@ -130,6 +130,7 @@ func (cl *Client) TorrentReadAt(ih InfoHash, off int64, p []byte) (n int, err er
 		err = io.EOF
 		return
 	}
+	t.lastReadPiece = int(index)
 	piece := t.Pieces[index]
 	if !piece.EverHashed {
 		cl.queuePieceCheck(t, index)
@@ -714,8 +715,26 @@ type ResponsiveDownloadStrategy struct{}
 func (ResponsiveDownloadStrategy) FillRequests(t *torrent, c *connection) {
 	for e := t.Priorities.Front(); e != nil; e = e.Next() {
 		if !c.Request(e.Value.(request)) {
+			return
+		}
+	}
+	for i := t.lastReadPiece; i < t.lastReadPiece+5 && i < t.NumPieces(); i++ {
+		for cs := range t.Pieces[i].PendingChunkSpecs {
+			if !c.Request(request{pp.Integer(i), cs}) {
+				return
+			}
+		}
+	}
+	// Then finish off incomplete pieces in order of bytes remaining.
+	for _, index := range t.piecesByPendingBytes() {
+		if t.PieceNumPendingBytes(index) == t.PieceLength(index) {
 			break
 		}
+		for chunkSpec := range t.Pieces[index].PendingChunkSpecs {
+			if !c.Request(request{index, chunkSpec}) {
+				return
+			}
+		}
 	}
 }
 

torrent.go

@@ -35,6 +35,7 @@ type torrent struct {
 	// BEP 12 Multitracker Metadata Extension. The tracker.Client instances
 	// mirror their respective URLs from the announce-list key.
 	Trackers      [][]tracker.Client
+	lastReadPiece int
 }
 
 func (t *torrent) String() string {