nghttpx: Call upstream resume_read after sending pending request to backend

With the combination of HTTP/1 upstream and HTTP/2 downstream,
downstream tells SHRPX_NO_BUFFER while connecting to the backend
server.  Previously, we did not call upstream resume_read and upload
was blocked.  This commit now calls upstream resume_read to unblock.
This commit also remove pending output buffer size of Http2Session
when calculating downstream connection's buffer is full.  This is
desirable since we only operate resume_read by stream basis.

src/shrpx_http2_downstream_connection.cc

@@ -583,7 +583,7 @@ StreamData* Http2DownstreamConnection::detach_stream_data()
 bool Http2DownstreamConnection::get_output_buffer_full()
 {
   if(request_body_buf_) {
-    return http2session_->get_outbuf_length() +
+    return
       evbuffer_get_length(request_body_buf_) >= Http2Session::OUTBUF_MAX_THRES;
   } else {
     return false;

src/shrpx_http2_session.cc

@@ -1471,6 +1471,9 @@ int Http2Session::on_connect()
   // submit pending request
   for(auto dconn : dconns_) {
     if(dconn->push_request_headers() == 0) {
+      auto downstream = dconn->get_downstream();
+      auto upstream = downstream->get_upstream();
+      upstream->resume_read(SHRPX_NO_BUFFER, downstream, 0);
       continue;
     }