[MINOR] http: add a separate "http-keep-alive" timeout

This one is used to wait for next request after a response was sent
to the client.
diff --git a/include/types/proxy.h b/include/types/proxy.h
index 78449e7..2ba298b 100644
--- a/include/types/proxy.h
+++ b/include/types/proxy.h
@@ -201,6 +201,7 @@
 		int server;                     /* server I/O timeout (in ticks) */
 		int appsession;                 /* appsession cookie expiration */
 		int httpreq;                    /* maximum time for complete HTTP request */
+		int httpka;                     /* maximum time for a new HTTP request when using keep-alive */
 		int check;                      /* maximum time for complete check */
 	} timeout;
 	char *id, *desc;			/* proxy id (name) and description */