BUG/MEDIUM: cli/shutdown sessions: make it thread-safe

BUG/MEDIUM: cli/shutdown sessions: make it thread-safe

There's no locking around the lookup of a stream nor its shutdown
when issuing "shutdown sessions" over the CLI so the risk of crashing
the process is particularly high.

Let's use a thread_isolate() there which is suitable for this task, and
there are not that many alternatives.

This must be backported to 1.8.

(cherry picked from commit 3f5dd2945ccbed6b4baf23a453e9d1d071a2d835)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
(cherry picked from commit 9ca4d7fb602ddc3c27e9e79c60627f4a054bf75f)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
(cherry picked from commit a4f1b27f5075e9bf881d1f329ad30a81e0f37650)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>