Implement the same event processing mechanism as in Java lower-level API

[dartvandru]

The lower-level API replaced acknowledgeable stream mechanism with a simple callback based API.

The dispatcherCallback returns a CompletableFuture (ie. Promise).
When it completes the implementation considers that message processed.
The order of messages to ACK must be preserved.I.e. the MESSAGE frames must be ACK’d in the order they are received.
For example, If there are messages A, B, and C then if the handler completes in this order C, B, A then the ACKs for B and C are not sent until A completes.

A MESSAGE frame’s ack header is an opaque token that should never be decoded - simply passed to the ACK frame.