Cannot maintain effective debug-level on distributed (parent) logger with local scheduler/cluster

With `config.yaml` including simple-form `logging` config:

```
logging:
distributed: debug

>>> import logging
>>> import distributed
>>> logging.getLogger("distributed")

>>> logging.getLogger("distributed").handlers
[ (NOTSET)>]
>>> logging.getLogger("distributed").debug("debug")
distributed - DEBUG - debug
>>>
```

All looks good. Start a `Client`:

```
>>> from distributed import Client
>>> c = Client()
>>> logging.getLogger("distributed")

>>> logging.getLogger("distributed").handlers
[ (WARNING)>]
>>> logging.getLogger("distributed").debug("debug")
>>>
```
No logging output, as the `distributed` logger has had its `Handler` (a `StreamHandler`) set to `WARNING` threshold.

Once a `Client` is started and fires-up a local cluster/scheduler, I suspect a worker:

https://github.com/dask/distributed/blob/09b959a5667a51a2dc073510c784e01d44827457/distributed/worker.py#L424-L425

... runs `silence_logging`, which does just as it is named:

https://github.com/dask/distributed/blob/09b959a5667a51a2dc073510c784e01d44827457/distributed/utils.py#L728-L742

See also #2659 . It might be argued that nothing important is logging to the `distributed` logger, and everything is logging to a child e.g. `distributed.client` (even through the loggers are flattened with the simple config), which might be true, but it further erodes the notion of hierarchical logging.