DEBUG: stream: do not emit debug messages for internal proxies

DEBUG: stream: do not emit debug messages for internal proxies

The debug mode that is triggered via "haproxy -d" remains helpful in
unit tests during development, typically to see HTTP headers being
processed. However, for historic reasons it doesn't make any difference
between service connections and maintenance connections such as peers
or CLI, making it close to useless in environments involving one of
these.

This patch just eliminates logs for sessions coming from an internal
proxy, since this is not something users are expected to deal with nor
want to observe. Thus, it finally becomes possible to use "haproxy -d"
while a monitoring script polls the CLI or when peers are configured.