MEDIUM: stconn: Replace read and write timeouts by a unique I/O timeout

Read and write timeouts (.rto and .wto) are now replaced by an unique
timeout, call .ioto. Since the recent refactoring on channel's timeouts,
both use the same value, the client timeout on client side and the server
timeout on the server side. Thus, this part may be simplified. Now it
represents the I/O timeout.
diff --git a/src/http_client.c b/src/http_client.c
index 04e1367..a30a941 100644
--- a/src/http_client.c
+++ b/src/http_client.c
@@ -1059,8 +1059,7 @@
 	s = appctx_strm(appctx);
 	s->target = target;
 	/* set the "timeout server" */
-	s->scb->rto = hc->timeout_server;
-	s->scb->wto = hc->timeout_server;
+	s->scb->ioto = hc->timeout_server;
 
 	if (doresolve) {
 		/* in order to do the set-dst we need to put the address on the front */