feature: Option for batch to only contain messages of the same key

Currently the batches are just a subset of non batched processing.
I.e.
- key based ordering - all keys in all batches will be unique.
- Unordered - no restrictions

Imagine a scenario where you have a topic where key is the account.

- You have a database per account.
- You want to do batch inserts to the database, where you do a batch of inserts (one insert per message) from within the batch.
- You don't want any messages from a different key, in this batch. All messages in this batch will be in a single external system call.
- Ordered inserting in db is still required. Key ordering is used. No other batch in flight is allowed to contain keys already in an inflight batch.