Use the `trace` decorator everywhere

Change the `trace` decorator to log at DEBUG level instead of
monkey patching the logger to introduce a new LOG_LEVEL.

Also, remove redundant logging.
Why keep the format string away from the arguments? I'd argue

```python
return 'DiskFileWriter(filesystem=%r, object_name=%r)' % (self._filesystem, self._name)
```

is less error-prone (no separation between format string and number/order of interpolation arguments).