HTTP/2: fixed closed_nodes overflow (ticket #1708). With large http2_max_concurrent_streams or http2_max_concurrent_pushes, more than 255 ngx_http_v2_node_t structures might be allocated, eventually leading to h2c->closed_nodes overflow when closing corresponding streams. This will in turn result in additional allocations in ngx_http_v2_get_node_by_id(). While mostly harmless, it can result in excessive memory usage by a HTTP/2 connection, notably in configurations with many keepalive_requests allowed. Fix is to use ngx_uint_t for h2c->closed_nodes instead of unsigned:8.