BUG/MINOR: stconn: bs.id and fs.id had their dependencies incorrect

BUG/MINOR: stconn: bs.id and fs.id had their dependencies incorrect

The backend depends on the response and the frontend on the request, not
the other way around. In addition, they used to depend on L6 (hence
contents in the channel buffers) while they should only depend on L5
(permanent info known in the mux).

This came in 2.9 with commit 24059615a7 ("MINOR: Add sample fetches to
get the frontend and backend stream ID") so this can be backported there.

(cherry picked from commit 61dd0156c82ea051779e6524cad403871c31fc5a)
Signed-off-by: Willy Tarreau <w@1wt.eu>