Capturing and logging stdout/stderr on workers

For various reasons, it is nice to be able to use `stdout` and `stderr` on workers and see the results. This can be useful in simplistic debugging attempts, using 3rd party code that already `print`s to one (or both) of these, etc. However it appears `stdout` and `stderr` are ignored in these settings currently.

A very useful feature would be to capture `stdout` and `stderr` on the worker and redirect them to a logger. One simple option would be to use the loggers `distributed.worker.stdout` and `distributed.worker.stderr` with `info` level messages. These loggers seem to be unused now and redirecting to them seems to work fine. This would at least give users one reasonable way to get this information.