Remove support for asynchronous acks for RecvPacket

[plafer]

Asynchronous acknowledgements are currently under-specified. I don't understand the semantics enough to be confident that our current implementation is correct. Hence, it makes our implementation of RecvPacket more complex, while the benefits are unclear.

We should remove support for them and document it in our list of unsupported features. We can add support at a later time if requested, when the requirements are more clear.

[plafer]

Note that our current implementation didn't properly support async acks, since the packet processing callbacks have no way of calling write_acknowledgement::process(), which requires a reference to a ChannelReader.

The packet processing callbacks don't need a reference to the context; a chain can call write_acknowledgement::process() in the processing of a chain-specific transaction. This means that we'll need to make sure to expose a write_acknowledgement() function when we add support back for async acks.