Unify batching API

[astubbs]

See

• major: #18 Batch support and engine event system improvements #19
• Replace raw ConsumerRecord with read only context object wrapper #216

Currently there are separate methods for batching. This ticket is to remove the special batch API, and simply make batches a detail of the normal API.

Opposing view - that forces all user functions to take a List of records. However this would be made nicer after #216, as the change would at least be wrapped.

[astubbs]

done in #223