Give the broadcaster a shutdown path

[nickschuch]

The broadcaster's run goroutine looped forever with no way to stop, and Subscribe/Unsubscribe blocked on unbuffered channels handed to it. This was latent while nothing cancelled the process context, but now that a signal cancels it the sidecar's shutdown drives this path: server.Shutdown waits for the streaming handlers to return, and they only return once their subscriber channel closes.

Run now takes a context and, on cancellation, closes every subscriber channel and returns, so blocked stream handlers unblock and server.Shutdown completes instead of waiting out its timeout. Subscribe, Unsubscribe and ProcessTrace select on a done channel so they no longer block once the loop has stopped; Subscribe hands back a closed channel after shutdown. The broadcaster is built from the errgroup context, and the tests pass one that ends with the test.

Adds a shutdown test asserting subscribers are closed and the API stops blocking, and moves the existing tests onto a cancellable context.