Check if the particular key exists in the storage

    :param key: name of the key which existence need to be checked
    :return:

    :type key str
    :rtype bool

def exists(self, key):
    """
    Check if the particular key exists in the storage

    :param key: name of the key which existence need to be checked
    :return:

    :type key str
    :rtype bool
    """
    if key in self._keystore and not self._keystore[key].is_expired:
      return True
    elif key in self._keystore and self._keystore[key].is_expired:
      self.delete(key)
      return False

    return False