NetworkPkg: Stop and release DHCP4 child after boot info is ready

HttpBootDxe need to stop and release the DHCP4 child when it's
not used so the NBP could create new DHCP4 child and use it.

Cc: Ye Ting <ting.ye@intel.com>
Cc: Zhang Lubo <lubo.zhang@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Jiaxin Wu <jiaxin.wu@intel.com>
Reviewed-by: Ye Ting <ting.ye@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@18231 6f19259b-4bc3-4df7-8a09-765794883524