BUG/MEDIUM: server: close a race around ready_srv when deleting a server

BUG/MEDIUM: server: close a race around ready_srv when deleting a server

When a server is being disabled or deleted, in case it matches the
backend's ready_srv, this one is reset. However it's currently done in
a non-atomic way when the server goes down, and that could occasionally
reset the entry matching another server, but more importantly if in
parallel some requests are dequeued for that server, it may re-appear
there after having been removed, leading to a possible crash once it
is fully removed, as shown in issue #3177.

Let's make sure we reset the pointer when detaching the server from
the proxy, and use a CAS in both cases to only reset this server.

This fix needs to be backported to 3.2. There, srv_detach() is in
server.c instead of server.h. Thanks to Basha Mougamadou for the
detailed report and the useful backtraces.