Ensuring thread safety

In order to ensure thread safety, the ThrottleCounter object is now of type struct and all CRUD operations are done with locking.

static readonly object _processLocker = new object();
private ThrottleCounter ProcessRequest(
    ThrottlePolicy throttlePolicy, 
    RequestIndentity throttleEntry, 
    TimeSpan timeSpan, 
    RateLimitPeriod period, out string id)
{
    var throttleCounter = new ThrottleCounter();
    throttleCounter.Timestamp = DateTime.UtcNow;
    throttleCounter.TotalRequests = 1;

    id = "throttle";

    //computed request unique id from IP, client key, url and period

    //get the hash value of the computed id
    var hashId = ComputeHash(id);

    //serial reads and writes
    lock (_processLocker)
    {
        var entry = Repository.FirstOrDefault(hashId);
        if (entry.HasValue)
        {
            //entry has not expired
            if (entry.Value.Timestamp + timeSpan >= DateTime.UtcNow)
            {
                //increment request count
                var totalRequests = entry.Value.TotalRequests + 1;

                //deep copy
                throttleCounter = new ThrottleCounter
                {
                    Timestamp = entry.Value.Timestamp,
                    TotalRequests = totalRequests
                };
            }
        }

        //stores: id (string) - timestamp (datetime) - total (long)
        Repository.Save(hashId, throttleCounter, timeSpan);
    }

    return throttleCounter;
}