[MEDIUM] introduce "timeout http-request" in frontends

In order to offer DoS protection, it may be required to lower the maximum
accepted time to receive a complete HTTP request without affecting the client
timeout. This helps protecting against established connections on which
nothing is sent. The client timeout cannot offer a good protection against
this abuse because it is an inactivity timeout, which means that if the
attacker sends one character every now and then, the timeout will not
trigger. With the HTTP request timeout, no matter what speed the client
types, the request will be aborted if it does not complete in time.
diff --git a/include/types/proto_http.h b/include/types/proto_http.h
index c5e051e..6453061 100644
--- a/include/types/proto_http.h
+++ b/include/types/proto_http.h
@@ -241,6 +241,7 @@
 	char *srv_cookie;		/* cookie presented by the server, in capture mode */
 	int status;			/* HTTP status from the server, negative if from proxy */
 	unsigned int flags;             /* transaction flags */
+	struct timeval exp;             /* expiration date for the transaction (generally a request) */
 };
 
 /* This structure is used by http_find_header() to return values of headers.