BUG/MINOR: stream: fix age calculation in "show sess" output

BUG/MINOR: stream: fix age calculation in "show sess" output

The "show sess" output reports an age that's based on the last byte of
the HTTP request instead of the stream creation date, due to a confusion
between logs->request_ts and the request_date sample fetch function. Most
of the time these are equal except when the request is not yet full for
any reason (e.g. wait-body). This explains why a few "show sess" could
report a few new streams aged by 99 days for example.

Let's perform the correct request timestamp calculation like the sample
fetch function does, by adding t_idle and t_handshake to the accept_ts.
Now the stream's age is correct and can be correctly used with the
"show sess older <age>" variant.

This issue was introduced in 2.9 and the fix can be backported to 3.0.