NetworkPkg: Update cache management in HTTP boot driver.

The original HTTP boot driver always save the received message body in its cache,
it bring a large of memory allocation during HTTP download. This patch updates
the HTTP boot driver to only cache data when caller doesn't provide a buffer for
download (which is usually used when caller want to get the required buffer size).

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Fu Siyuan <siyuan.fu@intel.com>
Reviewed-by: Ye Ting <ting.ye@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@18448 6f19259b-4bc3-4df7-8a09-765794883524