Exception when many threads wait to acquire etcd3 lock

When 10 threads wait to acquire the etcd3 lock, I see this exception sometimes:

```
with etcd_client.lock("etcd_lock"):
File "/usr/local/lib/python2.7/dist-packages/etcd3/locks.py", line 136, in __enter__
self.acquire()
File "/usr/local/lib/python2.7/dist-packages/etcd3/locks.py", line 103, in acquire
return _acquire()
File "/usr/local/lib/python2.7/dist-packages/tenacity/__init__.py", line 231, in wrapped_f
return self.call(f, *args, **kw)
File "/usr/local/lib/python2.7/dist-packages/tenacity/__init__.py", line 313, in call
start_time=start_time)
File "/usr/local/lib/python2.7/dist-packages/tenacity/__init__.py", line 269, in iter
return fut.result()
File "/usr/local/lib/python2.7/dist-packages/concurrent/futures/_base.py", line 455, in result
return self.__get_result()
File "/usr/local/lib/python2.7/dist-packages/tenacity/__init__.py", line 316, in call
result = fn(*args, **kwargs)
File "/usr/local/lib/python2.7/dist-packages/etcd3/locks.py", line 83, in _acquire
self.lease = self.etcd_client.lease(self.ttl)
File "/usr/local/lib/python2.7/dist-packages/etcd3/client.py", line 48, in handler
_translate_exception(exc)
File "/usr/local/lib/python2.7/dist-packages/etcd3/client.py", line 32, in _translate_exception
raise exception
ConnectionFailedError
```

Start with the reported 10-thread reproduction and inspect etcd3/locks.py, especially acquire and _acquire, then follow the exception translation in etcd3/client.py. Determine why waiting threads can raise ConnectionFailedError and verify that the concurrent lock scenario no longer produces the reported exception.

Written by the indexing model from the issue text.