message: fix mongo db plugin compilation
diff: @@ -475,7 +475,9 @@ void mongo_db_plugin_impl::_process_block(const block_trace& bt, const signed_bl
  "unknown";
  trx_status_map[trx_trace.id] = trx_status;
  - for (const auto& trx : trx_trace.deferred_transactions) {
  + for (const auto& req : trx_trace.deferred_transaction_requests) {
  + if ( req.contains<chain::deferred_transaction>() ) {
  + auto trx = req.get<chain::deferred_transaction>();
  auto doc = process_trx(trx);
  doc.append(kvp("type", "deferred"),
  kvp("sender_id", b_int64{static_cast<int64_t>(trx.sender_id)}),
  @@ -485,6 +487,14 @@ void mongo_db_plugin_impl::_process_block(const block_trace& bt, const signed_bl
  mongocxx::model::insert_one insert_op{doc.view()};
  bulk_trans.append(insert_op);
  ++trx_num;
  + } else {
  + auto cancel = req.get<chain::deferred_reference>();
  + auto doc = bsoncxx::builder::basic::document{};
  + doc.append(kvp("type", "cancel_deferred"),
  + kvp("sender_id", b_int64{static_cast<int64_t>(cancel.sender_id)}),
  + kvp("sender", cancel.sender.to_string())
  + );
  + }
  }
  if (!trx_trace.action_traces.empty()) {
  actions_to_write = true;
  