move churn modules into this repository

Obsoletes the `uproxy-churn` repository:
https://github.com/uProxy/uproxy/issues/445

This mostly just copies the code wholesale but I took the opportunity to rename the modules a little (it's more important they have descriptive names when there's a bunch living side by side):
* `turn` and `net` become `turn-frontend` and `turn-backend`, respectively
* `pipe` becomes `churn-pipe`

Unit tests pass and I tested all the sample apps manually.

Fewer repositories for all!
https://github.com/uProxy/uproxy/issues/507