BUG/MEDIUM: backend: never reuse a connection for tcp mode

BUG/MEDIUM: backend: never reuse a connection for tcp mode

The reuse of idle connections should only happen for a proxy with the
http mode. In case of a backend with the tcp mode, the reuse selection
and insertion in session list are skipped.

This behavior is present since commit :
MEDIUM: connection: Add private connections synchronously in session server list
It could also be further exagerated by :
MEDIUM: backend: add reused conn to sess if mux marked as HOL blocking

It can be backported up to 2.3.

(cherry picked from commit 08d87b3f49867440f66aee09173c84bf58cbc859)
Signed-off-by: Willy Tarreau <w@1wt.eu>