dhcp6: start request when advertise received after IRT (#376)

dhcp6: start request when advertise received after IRT (#376)

After the initial solicit timeout, any received advertisements trigger
a request.  However, after the timeout, any advertisements will
never result in a request, and the client sends solicits forever.

This patch adds sends a request in response to an advertise if the
initial timeout has expired.

Cleanup: removes a check for an impossible state.