BUG/MAJOR: server: fix deadlock when changing maxconn via agent-check

The server_parse_maxconn_change_request locks the server lock. However,
this function can be called via agent-checks or lua code which already
lock it. This bug has been introduced by the following commit :

  commit 79a88ba3d0
  BUG/MAJOR: server: prevent deadlock when using 'set maxconn server'

This commit tried to fix another deadlock with can occur because
previoulsy server_parse_maxconn_change_request requires the server lock
to be held. However, it may call internally process_srv_queue which also
locks the server lock. The locking policy has thus been updated. The fix
is functional for the CLI 'set maxconn' but fails to address the
agent-check / lua counterparts.

This new issue is fixed in two steps :
- changes from the above commit have been reverted. This means that
  server_parse_maxconn_change_request must again be called with the
  server lock.

- to counter the deadlock fixed by the above commit, process_srv_queue
  now takes an argument to render the server locking optional if the
  caller already held it. This is only used by
  server_parse_maxconn_change_request.

The above commit was subject to backport up to 1.8. Thus this commit
must be backported in every release where it is already present.

include/haproxy/queue.h

@@ -34,7 +34,7 @@ extern struct pool_head *pool_head_pendconn;
 
 struct pendconn *pendconn_add(struct stream *strm);
 int pendconn_dequeue(struct stream *strm);
-void process_srv_queue(struct server *s);
+void process_srv_queue(struct server *s, int server_locked);
 unsigned int srv_dynamic_maxconn(const struct server *s);
 int pendconn_redistribute(struct server *s);
 int pendconn_grab_from_px(struct server *s);