MEDIUM: log: make sess_build_logline() support being called with no stream

MEDIUM: log: make sess_build_logline() support being called with no stream

Till now it was impossible to emit logs from the lower layers only because
a stream was mandatory. From now on it will at least be possible to emit a
log to report a bad request or some timings for example. When the stream
is null, sess_build_logline() will use default values and will extract the
timing information from the session just like stream_new() does, so the
resulting log line is perfectly valid.

The termination state will indicate a proxy error during the request phase
since it is the only realistic use for such a call with no stream.