[STATS] report the number of times each server was selected

[STATS] report the number of times each server was selected

One user reported that an indicator was missing in the statistics:
the number of times each server was selected by load balancing. It
is in fact the total number of sessions assigned to a server by the
load balancing algorithm. It should directly reflect the weight for
"fair" algorithms such as round-robin, since it will not account for
persistant connections.

It should help a lot tuning each server's weight depending on the
load it receives.