domain: ack messages with one byte to closer match fifo benchmark

[chzchzchz]

pipe/fifo benchmark signals but domain replies with full payload, which causes a significant performance difference.

[alexandervanrenen]

Hi @chzchzchz ,

Thanks for your pr. I checked your code and understand that there is a mismatch between the pipe/fifo and domain. However the domain-socket benchmark is actually the intended behavior. The full message should be send in both directions. We will have to fix this in pipe and fifo. I will create an issue.

Thanks, Alex