MINOR: tcp-sample: permit retrieving tcp_info from the connection/session stage

MINOR: tcp-sample: permit retrieving tcp_info from the connection/session stage

The fc_xxx info that are retrieved over tcp_info could currently not
be accessed before a stream is created due to a test that verified the
existence of a stream. The rationale here was that the function works
both for frontend and backend. Let's always retrieve these info from
the session for the frontend case so that it now becomes possible to
set variables at connection/session time. The doc did not mention this
limitation so this could almost be considered as a bug.