samples: net: coap_server: Finetune sample parameters

Finetune coap_server sample parameters for better user experience.

Reduce the maximum retransmission count to release resources earilier in
case client is unreachable and no longer responds. Increase the maximum
number of pending retransmissions, along with the heap size (the buffers
for messages are allocated on a heap), to improve sample responsiveness,
in case multiple unreplied messages are pending retransmission.

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