/// <summary>
/// Cleans the expired items from the cache.
/// </summary>
/// <remarks>CFI, 2012-03-10</remarks>
private void CleanExpired()
{
var expired = from l in lifetime.AsParallel()
where l.Value <= DateTimeOffset.Now
select l;
expired.ForAll((e) => { values.Remove(e.Key); lifetime.Remove(e.Key); });
}