sd-dhcp-client,-server: set timestamp based on the time when received a packet

sd-dhcp-client,-server: set timestamp based on the time when received a packet

It seems that RFC does not say anything about the timestamp of lease
we should use: time that the client sent a request or received a reply.
In DHCPv6 client and NDisc, we use a timestamp that we receive a packet,
rather than we sent something. So, let's consistently use the same
logic for DHCPv4 client.

By using the logic, we will hopefully not forget to set timestamp again,
which is fixed by 089362976c2a653a77f942bfeb3f61d0e180f078.