message: Add expiration to txn_test_gen_plugin txns
  Looks like txn_test_gen_plugin wasn't set expiration for a couple of its transactions. Previously that didn't matter, now it does. Also I had problems with stopping the transaction generation so fix that too.
diff: @@ -144,6 +144,7 @@ struct txn_test_gen_plugin_impl {
  trx.actions.emplace_back( vector<chain::permission_level>{{newaccountC,"active"}}, handler);
  }
  + trx.expiration = cc.head_block_time() + fc::seconds(30);
  trx.set_reference_block(cc.head_block_id());
  trx.sign(txn_test_receiver_C_priv_key, chainid);
  cc.push_transaction(packed_transaction(trx));
  @@ -187,6 +188,7 @@ struct txn_test_gen_plugin_impl {
  trx.actions.push_back(act);
  }
  + trx.expiration = cc.head_block_time() + fc::seconds(30);
  trx.set_reference_block(cc.head_block_id());
  trx.sign(txn_test_receiver_C_priv_key, chainid);
  cc.push_transaction(packed_transaction(trx));
  @@ -227,7 +229,7 @@ struct txn_test_gen_plugin_impl {
  void arm_timer(boost::asio::high_resolution_timer::time_point s) {
  timer.expires_at(s + std::chrono::milliseconds(timer_timeout));
  timer.async_wait([this](const boost::system::error_code& ec) {
  - if(ec)
  + if(!running || ec)
  return;
  try {
  send_transaction();
  