MdeModulePke/Mtftp4Dxe: Support windowsize in read request operation.

Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=886

This patch is to support the TFTP windowsize option described in RFC 7440.
The feature allows the client and server to negotiate a window size of
consecutive blocks to send as an alternative for replacing the single-block
lockstep schema.

Currently, the windowsize for write request operation is not supported since
there is no real use cases.

Cc: Ye Ting <ting.ye@intel.com>
Cc: Fu Siyuan <siyuan.fu@intel.com>
Cc: Shao Ming <ming.shao@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Wu Jiaxin <jiaxin.wu@intel.com>
Tested-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Fu Siyuan <siyuan.fu@intel.com>
Reviewed-by: Ye Ting <ting.ye@intel.com>

MdeModulePkg/Universal/Network/Mtftp4Dxe/Mtftp4Impl.h

@@ -8,6 +8,7 @@
   RFC2347 - TFTP Option Extension
   RFC2348 - TFTP Blocksize Option
   RFC2349 - TFTP Timeout Interval and Transfer Size Options
+  RFC7440 - TFTP Windowsize Option
 
 Copyright (c) 2006 - 2018, Intel Corporation. All rights reserved.<BR>
 This program and the accompanying materials
@@ -56,6 +57,7 @@ typedef struct _MTFTP4_PROTOCOL MTFTP4_PROTOCOL;
 #define MTFTP4_DEFAULT_TIMEOUT      3
 #define MTFTP4_DEFAULT_RETRY        5
 #define MTFTP4_DEFAULT_BLKSIZE      512
+#define MTFTP4_DEFAULT_WINDOWSIZE   1
 #define MTFTP4_TIME_TO_GETMAP       5
 
 #define MTFTP4_STATE_UNCONFIGED     0
@@ -121,6 +123,14 @@ struct _MTFTP4_PROTOCOL {
   UINT16                        LastBlock;
   LIST_ENTRY                    Blocks;
 
+  UINT16                        WindowSize;
+
+  //
+  // Record the total received block number and the already acked block number.
+  //
+  UINT64                        TotalBlock;
+  UINT64                        AckedBlock;
+
   //
   // The server's communication end point: IP and two ports. one for
   // initial request, one for its selected port.