D3845: worker: support more return types in posix worker

[Yuya Nishihara]

>   >   >   I'm not in love with pickle. Could we use json or cbor instead?
>   >   
>   >   Perhaps cbor is better since it can be streamed and the overhead is pretty
>   >   low. We have to keep the message size small since rfd/wfd is a multi-writer
>   >   pipe.
>   
>   It's been recommended to me that we avoid the streaming flavor of
>   cbor, so we'd probably just do one-shot messages.

I meant multiple one-shot messages can be serialized over the pipe. JSON parser
doesn't work in that way. Each message must be written atomically.