[MEDIUM] log: take the logged server name from the stream interface

[MEDIUM] log: take the logged server name from the stream interface

With HTTP keep-alive, logging the right server name will be quite
complex because the assigned server will possibly change before we log.
Also, when we want to log accesses to an applet, it's not easy because
the applet becomes NULL again before logging.

The logged server's name is now taken from the target stored in the
stream interface. That way we can log an applet, a server name, or we
could even log a proxy or anything else if we wanted to. Ideally the
session should contain a desired target which is the one which should
be logged.