message: use internal _push_transaction which assumes lock is held
  this fixes and
diff: @@ -380,7 +380,7 @@ signed_block chain_controller::_generate_block(
  _pending_transactions.clear();
  for( const auto& t : pending ) {
  try {
  - push_transaction( t );
  + _push_transaction( t );
  } catch ( ... ) {
  }
  }
  