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.
It returns 'instaneously'. Logs look like
```
Dec 18 14:11:29 localhost object-server: ==> __iter__ (0): call {'self': DiskFileReader(filesystem=SproxydFileSystem(conn_timeout=10, proxy_timeout=3, base_path='/proxy/chord_path/', hosts_list=set([('172.24.4.3', 81)])), object_name='TEMPAUTH_test/toto/etc/hosts')} (txn: txf7acfd31e414445b8b7f2-005492d281)
Dec 18 14:11:29 localhost object-server: <== __iter__ (0): return <generator object __iter__ at 0x7f462632ce60> (txn: txf7acfd31e414445b8b7f2-005492d281)
```

I understand what you are saying. At least with this we know that an attempt to read the object was made. It does not provide much info, but it shows the executed code path.