MINOR: server: make srv_shutdown_sessions() call pendconn_redistribute()

MINOR: server: make srv_shutdown_sessions() call pendconn_redistribute()

When shutting down server sessions, the queue was not considered, which
is a problem if some element reached the queue at the moment the server
was going down, because there will be no more requests to kick them out
of it. Let's always make sure we scan the queue to kick these streams
out of it and that they can possibly find a more suitable server. This
may make a difference in the time it takes to shut down a server on the
CLI when lots of servers are in the queue.

It might be interesting to backport this to 3.0 but probably not much
further.

(cherry picked from commit 1d403caf8aa59c9070f30ea16017261cab679fe2)
Signed-off-by: Willy Tarreau <w@1wt.eu>