OPTIM: session: don't process the whole session when only timers need a refresh

OPTIM: session: don't process the whole session when only timers need a refresh

Having a global expiration timer for a task means that the tasks are regularly
woken up (at least after each expiration timer). It's totally useless and counter
productive to process the whole session upon each such wakeup, and it's fairly
easy to detect such wakeups, so let's just update the task's timer and return
to sleep when this happens.

For 100k concurrent connections with 10s of timeouts, this can save 10k wakeups
per second, which is not bad.