docs: private synchronized void fillPool()
     {
        final int connectionsToAdd = Math.min(config.getMaximumPoolSize() - getTotalConnections(), config.getMinimumIdle() - getIdleConnections())
                                     - addConnectionQueue.size();
        for (int i = 0; i < connectionsToAdd; i++) {
           addConnectionExecutor.submit((i < connectionsToAdd - 1) ? poolEntryCreator : postFillPoolEntryCreator);
        }
     }