BUG/MINOR: stats: last session was not always set

BUG/MINOR: stats: last session was not always set

Cyril Bonté reported that the "lastsess" field of a stats-only backend
was never updated. In fact the same is true for any applet and anything
not a server. Also, lastsess was not updated for a server reusing its
connection for a new request.

Since the goal of this field is to report recent activity, it's better
to ensure that all accesses are reported. The call has been moved to
the code validating the session establishment instead, since everything
passes there.