Returns the cipher for self.server_name.

def _GetServerCipher(self):
    """Returns the cipher for self.server_name."""

    if self.server_cipher is not None:
      expiry = self.server_cipher_age + rdfvalue.Duration("1d")
      if expiry > rdfvalue.RDFDatetime.Now():
        return self.server_cipher

    remote_public_key = self._GetRemotePublicKey(self.server_name)
    self.server_cipher = Cipher(self.common_name, self.private_key,
                                remote_public_key)
    self.server_cipher_age = rdfvalue.RDFDatetime.Now()
    return self.server_cipher