Remove Stream impl for DirectoryMonitor

[ranweiler]

The manual implementation of Stream for DirectoryMonitor misused the Waker provided by poll_next(). In practice, this resulted in telling the executor to poll the stream again immediately after any non-terminal try_recv(). The end result was equivalent to calling try_recv() in a tight loop. This pegged agent CPU usage as soon as we started monitoring directories for new files.

Now, we take a much simpler approach. The private into_async() shim function spawns a thread that blocks on new (synchronous) Receiver messages, and then immediately sends them to an unbounded async Receiver. All subsequent API surface can thus be effortlessly async.

DirectoryMonitor now exports a public next_file() async method, which is equivalent in practice to implementing Stream. We aren't using any Stream combinators, we don't lose anything in doing this. We can also use this to impl Stream again later, if needed.

• Locally tested that agent directory monitoring use sites pick up new files
• Confirmed that agent CPU usage drops from 100% to < 1%.
• Manually tested tasks on deployed instance