Source async commit/ack

[alexec]

Currently, the architecture essentially assumes that messages are sunk synchronously. This is not what we want for several reasons:

• Async Kafka producer.
• Prevents buffering of sinking.

But note:

• Some sources do not have ack, e.g. cron
• Some sources must be acks sync, e.g. HTTP
• Some sinks do not support async, e.g. HTTP
• Some sinks only support async, e.g. log (yes, log writes are async and can be lost)
• Some messages will not be sunk, e.g. due to filter op.

[github-actions]

Stale issue message