Stand By Feed and ChangeFeed pull model

[ealsur]

Poll model iterator for Change Feed

This PR introduces a GetStandByFeedIterator to CosmosItems that will sequentially read the Change Feed for a particular Container.

The sequential read will be done at the PartitionKeyRange level, reading each range until a 304 is obtained and continuing with the next range across FetchNextAsync calls.

In each iteration, a ContinuationToken will be returned that is a composite of a list of continuation tokens for all existing ranges.

The HasMoreResults variable is expected to be always true and is on the caller's responsibility to break out of the loop based on a particular logic (ie. when all ranges have been visited or when # amount of ranges have been visited).

Internally, the Iterator will handle the following error scenarios:

1. If the current range being read splits, a refresh of the children will be done and added to the continuation token for continuous read. Current range will be updated for a next call to FetchNextAsync.
2. If the current request exceeds Cosmos DB max response size (4Mb) due to the MaxItemSize parameter, the caller will be signaled to retry with HasMoreResults true in the response and internally, updating MaxItemSize to half.
3. If the current range does not exist anymore, it will be discarded from the token, and the next one will be available to read through a FetchNextAsync call (HasMoreResults will be true).

Additionally, when the user provides it's own ContinuationToken (obtained from a previous usage), the following validations will be done:

1. If the first range to read is invalid, it will be discarded, and the next valid one will be updated in the response's Continuation, ready for a FetchNextAsync.
2. If all ranges in the provided token are invalid/do not exist, a new token will be built based on the current PK Range distribution in the container will be generated and used.

Sample usage

CosmosItemsCore itemsCore = (CosmosItemsCore)this.Container.Items;
CosmosFeedResultSetIterator setIterator =
    itemsCore.GetStandByFeedIterator(new CosmosChangeFeedRequestOptions() {
       StartTime = DateTime.MinValue
});
while (setIterator.HasMoreResults)
{
    using (CosmosResponseMessage iterator =
        await setIterator.FetchNextSetAsync(this.cancellationToken))
    {
        Trace.WriteLine(iterator.Header.Continuation); // Global Continuation Token
        if (iterator.Content != null)
        {
            Trace.WriteLine(response.Count);
        }
    }
}

[kirankumarkolli]

Please check comments

[kirankumarkolli]

Sample usage refresh:
CosmosItemsCore itemsCore = (CosmosItemsCore)this.Container.Items;
CosmosFeedResultSetIterator setIterator =
itemsCore.GetStandByFeedIterator(new CosmosChangeFeedRequestOptions() {StartFromBeginning = true});

[j82w]

In the future it might be nice to include a rough design overview mark down file.

[j82w]

Since change feed is no longer part of the query pipeline maybe we should move it to a new namespace.

[kirankumarkolli]

Thanks alot @ealsur.