Today, after reading in a changeEvent from the cursor in the I/O thread, it is buffered and then later processed by the executor thread. Only the changeEvent is passed on. Add functionality to also pass on some per event metadata. Also consider offloading some functionality like transforming the BSON into a Document onto the I/O thread itself

In the context of initialSync, we need the ability to pass on a partitionId field per document as the metadata for that document.