NetworkPkg: Define one PCD for PXE to specify MTFTP windowsize.

NetworkPkg: Define one PCD for PXE to specify MTFTP windowsize.

*v2: Rename and redefine the PCD in NetworkPkg instead of MdeModulePkg.

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

This patch is to define one new PCD for PXE driver to specify MTFTP windowsize so as
to improve the PXE download performance. The default value is set to 4.

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