sd-dhcp6-client: Add Request message sending

sd-dhcp6-client: Add Request message sending

As described in RFC 3315, Section 17.1.2, a client has to wait until the
first timeout has elapsed before it is allowed to request IPv6 addresses
from the DHCPv6 server. This is indicated by a non-NULL lease and a
non-zero resend count. Should the Advertisement contain a preference
value of 255 or be received after the first timeout, IPv6 address
requesting is started immediately.

In response to these events, create a Request message and set up proper
resend timers to send the message to the server.