def driver_connection_pool(driver_class, driver_config)
      pool = search_driver_connection_pool(driver_class, driver_config)
      return (pool.nil?) ? set_driver_connection_pool(driver_class, driver_config) : pool
    end
def driver_connection_pool(driver_class, driver_config)
      pool = search_driver_connection_pool(driver_class, driver_config)
      return (pool.nil?) ? set_driver_connection_pool(driver_class, driver_config) : pool
    end
get driver connection pool
 if doesn't exist pool, it's made newly.

 @param driver_class   [RailsKvsDriver::Base]  driver_class
 @param driver_config  [Hash]                  driver_config
 @return [ConnectionPool] connection pool of driver