Support batch publishes

[ripienaar]

Proposed change

We need to support batch publishes for a few use cases:

• Clients want a very high ingest rate on streams and so might want to send batches of messages that would result in a single ACK
• We want to support almost transaction-like related writes where several publishes to a stream are treated as a group and that groups messages are all committed or not with no chance for half commits.

Use case

For typed KV we might expand a hash/map into many keys inside a bucket. Imagine a USER record holding name, address etc. The address would be 4 or 5 keys and when the address change all 5 keys MUST update or none of the 5 should update.

A batch feature will facilitate that.

Additional to the high throughput publish mode already mentioned, we should aim for orders of magnitude increase in write performance.