sd-dhcp6: ensure canceling lease timers and refactor setting lease

Factor our common code to set/clear the client lease.
Resetting the lease involves canceling old timers.

Also, most importantly, when resetting the lease
in client_receive_advertise(), ensure to cancel
the timers too. It is not immediately clear, why
at that point no timers would be pending.

The lease-unref from sd_dhcp6_client_unref() can
be omited, because of the preceeding client_reset()
call.