nghttpx: Feed read event rather than calling on_read

on_read may fail, but we failed to check its return value most of the
places.  This is because failure means deletion of ClientHandler, but
because of architecture, we cannot delete it.  Feeding read event is
better since we can move call on_read from libev callback.  We can
delete ClientHandler form there.

src/shrpx_https_upstream.cc

@@ -699,7 +699,10 @@ int HttpsUpstream::resume_read(IOCtrlReason reason, Downstream *downstream,
     // Process remaining data in input buffer here because these bytes
     // are not notified by readcb until new data arrive.
     http_parser_pause(&htp_, 0);
-    return on_read();
+
+    auto conn = handler_->get_connection();
+    ev_feed_event(conn->loop, &conn->rev, EV_READ);
+    return 0;
   }
 
   return 0;