Writes a new client startup record.

def WriteClientStartupInfo(self, client_id, startup_info):
    """Writes a new client startup record."""
    if client_id not in self.metadatas:
      raise db.UnknownClientError(client_id)

    ts = rdfvalue.RDFDatetime.Now()
    self.metadatas[client_id]["startup_info_timestamp"] = ts
    history = self.startup_history.setdefault(client_id, {})
    history[ts] = startup_info.SerializeToString()