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/server.h b/include/types/server.h
index 7c6d225..7d0ba45 100644
--- a/include/types/server.h
+++ b/include/types/server.h
@@ -210,6 +210,7 @@
 	int cur_sess;				/* number of currently active sessions (including syn_sent) */
 	unsigned maxconn, minconn;		/* max # of active sessions (0 = unlimited), min# for dynamic limit. */
 	int nbpend;				/* number of pending connections */
+	unsigned int queue_idx;			/* count of pending connections which have been de-queued */
 	int maxqueue;				/* maximum number of pending connections allowed */
 	struct freq_ctr sess_per_sec;		/* sessions per second on this server */
 	struct be_counters counters;		/* statistics counters */