void MarkAsCrowled(ActivityStream stream)
{
if (crawledStreams.ContainsKey(stream.StreamId))
{
if (crawledStreams[stream.StreamId] < stream.ExpiresAt)
crawledStreams[stream.StreamId] = stream.ExpiresAt;
}
else
crawledStreams.Add(stream.StreamId, stream.ExpiresAt);
}