MEDIUM: session: don't apply the retry delay when redispatching

MEDIUM: session: don't apply the retry delay when redispatching

The retry delay is only useful when sticking to a same server. During
a redispatch, it's useless and counter-productive if we're sure to
switch to another server, which is almost guaranteed when there's
more than one server and the balancing algorithm is round robin, so
better not pass via the turn-around state in this case. It could be
done as well for leastconn, but there's a risk of always killing the
delay after the recovery of a server in a farm where it's almost
guaranteed to take most incoming traffic. So better only kill the
delay when using round robin.