sd-dhcp-server: improve predictability of leases

sd-dhcp-server: improve predictability of leases

Rather than having all clients attempt to get the same leases (starting at the
beginning of the pool), make each client star at a random offset into the pool
determined by their client id. This greatly increases the chances of a given
client receiving the same IP address even though both the client and server
have lost any lease information (and distinct server instances handing out
the same leases).