Lease refresh issues when running etcd proxy

When running the etcd proxy and connecting python-etcd3 to it i can't refresh leases anymore.

Environment:
python: 2.7
python-etcd3: 0.8.1
etcd/etcd proxy: 3.3.8

Code:
``` python
client = etcd3.client(host='127.0.0.1', port=23790)
lease = client.lease(60)
lease.refresh()
```

Error:
```
lease.refresh()
Traceback (most recent call last):
File "", line 1, in
File "/opt/bin/active_python/lib/python2.7/site-packages/etcd3/leases.py", line 24, in refresh
return list(self.etcd_client.refresh_lease(self.id))
File "/opt/bin/active_python/lib/python2.7/site-packages/etcd3/client.py", line 42, in handler
_translate_exception(exc)
File "/opt/bin/active_python/lib/python2.7/site-packages/etcd3/client.py", line 39, in handler
for data in f(*args, **kwargs):
File "/opt/bin/active_python/lib/python2.7/site-packages/etcd3/client.py", line 688, in refresh_lease
credentials=self.call_credentials):
File "/opt/bin/active_python/lib/python2.7/site-packages/grpc/_channel.py", line 366, in next
return self._next()
File "/opt/bin/active_python/lib/python2.7/site-packages/grpc/_channel.py", line 357, in _next
raise self
grpc._channel._Rendezvous: <_Rendezvous of RPC that terminated with (StatusCode.CANCELLED, context canceled)>
```

The perl client also seems to have the same issue: https://github.com/etcd-io/etcd/issues/9751

Start with etcd3/leases.py and etcd3/client.py, especially refresh() and refresh_lease(), then reproduce the lease refresh through an etcd proxy using the reported Python 2.7 and etcd 3.3.8 setup. Done means lease.refresh() completes successfully through the proxy, with a regression test covering that path.

Written by the indexing model from the issue text.