Writes flow output plugin log entries for a given flow.

def WriteFlowLogEntries(self, entries):
    """Writes flow output plugin log entries for a given flow."""
    flow_ids = [(e.client_id, e.flow_id) for e in entries]
    for f in flow_ids:
      if f not in self.flows:
        raise db.AtLeastOneUnknownFlowError(flow_ids)

    for e in entries:
      dest = self.flow_log_entries.setdefault((e.client_id, e.flow_id), [])
      to_write = e.Copy()
      to_write.timestamp = rdfvalue.RDFDatetime.Now()
      dest.append(to_write)