sd-dhcp-server: use sd_dhcp_message to parse received messages

sd-dhcp-server: use sd_dhcp_message to parse received messages

This is mostly refactoring. This does not change basic behavior, but
changes/fixes some minor/corner cases, e.g.

- extend the minimum default lease time from 1 second to 30 seconds, as
  1 second is too short and causes the network unstable (though 30
  seconds is stll too short, but hopefully that does not make the
  network unstable).
- error code on broken/malicious message received may be changed.