MINOR: http: implement the max-keep-alive-queue setting

Finn Arne Gangstad suggested that we should have the ability to break
keep-alive when the target server has reached its maxconn and that a
number of connections are present in the queue. After some discussion
around his proposed patch, the following solution was suggested : have
a per-proxy setting to fix a limit to the number of queued connections
on a server after which we break keep-alive. This ensures that even in
high latency networks where keep-alive is beneficial, we try to find a
different server.

This patch is partially based on his original proposal and implements
this configurable threshold.
diff --git a/include/types/proxy.h b/include/types/proxy.h
index 28be984..4c0c660 100644
--- a/include/types/proxy.h
+++ b/include/types/proxy.h
@@ -289,6 +289,7 @@
 	struct list pendconns;			/* pending connections with no server assigned yet */
 	int nbpend;				/* number of pending connections with no server assigned yet */
 	int totpend;				/* total number of pending connections on this instance (for stats) */
+	int max_ka_queue;			/* 1+maximum requests in queue accepted for reusing a K-A conn (0=none) */
 	unsigned int feconn, beconn;		/* # of active frontend and backends sessions */
 	struct freq_ctr fe_req_per_sec;		/* HTTP requests per second on the frontend */
 	struct freq_ctr fe_conn_per_sec;	/* received connections per second on the frontend */