sftp: add --sftp-chunk-size to control packets sizes for high latency links

See: https://forum.rclone.org/t/increasing-sftp-transfer-speed/29928

backend/sftp/sftp.go

@@ -272,6 +272,26 @@ given, rclone will empty the connection pool.
 Set to 0 to keep connections indefinitely.
 `,
 			Advanced: true,
+		}, {
+			Name: "chunk_size",
+			Help: `Upload and download chunk size.
+
+This controls the maximum packet size used in the SFTP protocol. The
+RFC limits this to 32768 bytes (32k), however a lot of servers
+support larger sizes and setting it larger will increase transfer
+speed dramatically on high latency links.
+
+Only use a setting higher than 32k if you always connect to the same
+server or after sufficiently broad testing.
+
+For example using the value of 252k with OpenSSH works well with its
+maximum packet size of 256k.
+
+If you get the error "failed to send packet header: EOF" when copying
+a large file, try lowering this number.
+`,
+			Default:  32 * fs.Kibi,
+			Advanced: true,
 		}},
 	}
 	fs.Register(fsi)
@@ -304,6 +324,7 @@ type Options struct {
 	DisableConcurrentReads  bool          `config:"disable_concurrent_reads"`
 	DisableConcurrentWrites bool          `config:"disable_concurrent_writes"`
 	IdleTimeout             fs.Duration   `config:"idle_timeout"`
+	ChunkSize               fs.SizeSuffix `config:"chunk_size"`
 }
 
 // Fs stores the interface to the remote SFTP files
@@ -481,6 +502,7 @@ func (f *Fs) newSftpClient(conn *ssh.Client, opts ...sftp.ClientOption) (*sftp.C
 		sftp.UseFstat(f.opt.UseFstat),
 		sftp.UseConcurrentReads(!f.opt.DisableConcurrentReads),
 		sftp.UseConcurrentWrites(!f.opt.DisableConcurrentWrites),
+		sftp.MaxPacketUnchecked(int(f.opt.ChunkSize)),
 	)
 	return sftp.NewClientPipe(pr, pw, opts...)
 }