* op-batcher: add logging of compression algo

* add node logging of algo, add brotli default algo

* fix typos

* only log compression algo if present

* add type conversion abstraction to Batch interface

Since we're dealing now with wrapped types around Batch implementations,
this let's us transparently unwrap the underlying batch types. It makes
sense to add this to the interface, because getting the underlying types
from the interface is done in several places, so it's part of the
interface's contract.

* adapt BatchReader test