net: dhcpv4: Reimplement RENEW/REBIND logic according to RFC2131

DHCP Request retransmission in RENEW and REBIND states was not
compliant with RFC2131.

The retransmission interval should not be calculated as in REQUESTING
state in such case, but rather calculated based on the remaining T2
or lease time (depending on current state).

RFC doesn't also mention any retransmission count limit for those
states. The client should retransmit the REQUEST until T2 or lease
expiry time are reached.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>