This PR allows users to register a user-defined callback, which is invoked when records in the hybrid log become read-only. It can be used for various reasons:

(1) to perform data write-through to an external store
(2) to perform stream processing on the change log created by the key-value store

How it Works

The FASTER log accessor now supports the IObservable<IFasterScanIterator<Key, Value>> interface. This means you can Subscribe to FASTER log, in order to receive (as push calls) updates whenever records enter the read-only region of the hybrid log:

        var s = fht.Log.Subscribe(new LogObserver());
        ...
        s.Dispose();

The parameter LogObserver is an instance of type IObserver<IFasterScanIterator<Key, Value>>. User gets push-based callbacks via the OnNext method of the observer. Each callback provides an iterator for a batch of records that have just become read-only, in bulk. The user can then perform operations such as stream processing or write-through to the remote database.

Here is an example of the callback:

        class LogObserver : IObserver<IFasterScanIterator<Key, Value>>
        {
            public void OnCompleted()
            {
            }

            public void OnError(Exception error)
            {
            }

            public void OnNext(IFasterScanIterator<Key, Value> iter)
            {
                while (iter.GetNext(out RecordInfo info, out Key key, out Value value))
                {
                    // Process key and value
                }
                iter.Dispose();
            }
        }