Fix bug #4612: session_shared returns diagnostic 2

Fix bug #4612: session_shared returns diagnostic 2

The problem was that a session was marked "in use" in cases where there
were no network activity on target (caching , research).. MP would update
its timestamp of last use when it was not really used.. MP now only marks
last use timestamp on network activity. The expire loop now waits
session_ttl / 3 seconds - was 30 before - to ensure that configurations
with low session ttl's will expire properly.