Use Plax mother channel and Plax channel types

To provide some basic protocol support, we could use the [Plax](https://github.com/Comcast/plax) ["mother channel"](https://github.com/Comcast/plax/blob/main/doc/chan_mother.md) and various Plax channel [types](https://github.com/Comcast/plax/tree/main/chans). In particular, [`sio`](sio) could use this gear instead of providing its own versions.