OPTIM: server: switch the actconn list to an mt-list

OPTIM: server: switch the actconn list to an mt-list

The remaining contention on the server lock solely comes from
sess_change_server() which takes the lock to add and remove a
stream from the server's actconn list. This is both expensive
and pointless since we have mt-lists, and this list is only
used by the CLI's "shutdown server sessions" command!

Let's migrate to an mt-list and remove the need for this costly
lock. By doing so, the request rate increased by ~1.8%.

(cherry picked from commit 751153e0f119bec90455cda95166f1b29d8b0326)
[wt: the contention is extreme on high threads counts, thus this
     actconn series is backported; ctx adjustments]
Signed-off-by: Willy Tarreau <w@1wt.eu>