ice: increase stun RTO

This patch increases STUN RTO to 500ms.
This gives a better total time to check all ICE connections
(during negotiation). The previous RTO let less than 8s, too short
for ICE messages exchange over DHT.

Change-Id: I8088abdc4d2306931adb3fdc19edda290479ab6f
Tuleap: #614