dhcpv6-ia: rework reconfigure accept logic

dhcpv6-ia: rework reconfigure accept logic

Rework reconfigure accept retransmission logic; every DHCPv6 assignment
has its own reconfigure accept timer now making sure the timeout and
retransmission behavior is in line with RFC3315 paragraph 19.1.2.

While at it rename the original reconf_timer into valid_until_timeout as
it will only check now the lifetime of the assignments.

Set reconf_cnt to INT_MAX for every assignment which belongs to the same
client identifier; before the reconf_cnt of the first handled assignment
was set by mistake breaking the retransmission logic.

Signed-off-by: Hans Dedecker <dedeckeh@gmail.com>