MINOR: queue: store the queue index in the stream when enqueuing

We store the queue index in the stream and check it on dequeueing to
figure how many entries were processed in between. This way we'll be
able to count the elements that may later be added before ours.
diff --git a/include/types/queue.h b/include/types/queue.h
index c025b9c..575cc59 100644
--- a/include/types/queue.h
+++ b/include/types/queue.h
@@ -32,6 +32,7 @@
 
 struct pendconn {
 	int            strm_flags; /* stream flags */
+	unsigned int   queue_idx;  /* value of proxy/server queue_idx at time of enqueue */
 	struct stream *strm;
 	struct proxy  *px;
 	struct server *srv;        /* the server we are waiting for, may be NULL if don't care */