samples: net: coap_server: Fix retransmission mechanism

The retransmission handler only increased the retransmission count, w/o
sending the actual message again. Additionally, the next retransmission
time calculation was broken - it did not take into consideration the
time that has already passed since before retransmission, and re-applied
the entire timeout value.

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