HTTP/3: graceful shutdown on keepalive timeout expiration.

Previously, the expiration caused QUIC connection finalization even if
there are application-terminated streams finishing sending data.  Such
finalization terminated these streams.

An easy way to trigger this is to request a large file from HTTP/3 over
a small MTU.  In this case keepalive timeout expiration may abruptly
terminate the request stream.

src/http/v3/ngx_http_v3.c

@@ -70,7 +70,7 @@ ngx_http_v3_keepalive_handler(ngx_event_t *ev)
 
     ngx_log_debug0(NGX_LOG_DEBUG_HTTP, c->log, 0, "http3 keepalive handler");
 
-    ngx_http_v3_finalize_connection(c, NGX_HTTP_V3_ERR_NO_ERROR,
+    ngx_http_v3_shutdown_connection(c, NGX_HTTP_V3_ERR_NO_ERROR,
                                     "keepalive timeout");
 }