object-server logger can complain about missing txn_id

Seen in prod:

Traceback (most recent call last):
  File ".../swift/obj/server.py", line 1338, in __call__
    res = getattr(self, req.method)(req)
  File ".../swift/common/utils.py", line 2016, in _timing_stats
    resp = func(ctrl, *args, **kwargs)
  File ".../swift/obj/server.py", line 1047, in PUT
    writer.put(metadata)
  File ".../swift/obj/diskfile.py", line 2941, in put
    super(DiskFileWriter, self)._put(metadata, True)
  File ".../swift/obj/diskfile.py", line 1912, in _put
    tpool.execute(self._finalize_put, metadata, target_path, cleanup)
  File ".../eventlet/tpool.py", line 129, in execute
    six.reraise(c, e, tb)
  File ".../eventlet/tpool.py", line 83, in tworker
    rv = meth(*args, **kwargs)
  File ".../swift/obj/diskfile.py", line 1868, in _finalize_put
    target_path, new_target_path)
  File ".../logging/__init__.py", line 1446, in debug
    msg, kwargs = self.process(msg, kwargs)
  File ".../swift/common/utils.py", line 2165, in process
    kwargs['extra'] = {'server': self.server, 'txn_id': self.txn_id,
  File ".../swift/common/utils.py", line 2135, in txn_id
    return self._cls_thread_local.txn_id
  File ".../eventlet/corolocal.py", line 45, in __getattribute__
    return object.__getattribute__(self, attr)
AttributeError: 'local' object has no attribute 'txn_id' (txn: txa5431800f4ad4a1da42b5-00606bcb94)

*Super* weird that the log line about not having a txn_id *has a txn_id*.

I think this comes down to the same sorts of issues we've seen in the proxy that led to us plumbing logger_thread_locals through every time we use tpool.execute(), but we've *also* seen troubles with *that*, so... I'm not sure it's a good pattern to follow.