client support reconnection ？

connected to server and few minutes later call client.get('/xxx'), got error:

```
grpc._channel._Rendezvous: <_Rendezvous of RPC that terminated with:
status = StatusCode.UNAUTHENTICATED
details = "etcdserver: invalid auth token"
debug_error_string = "{"created":"@1540208787.330001000","description":"Error received from peer","file":"src/core/lib/surface/call.cc","file_line":1099,"grpc_message":"etcdserver: invalid auth token","grpc_status":16}"
```

Start by reproducing the reported sequence: connect to the server, wait several minutes, then call client.get('/xxx') and observe the invalid auth token error. Inspect the client's reconnection and authentication handling; done means the delayed request no longer fails because of an expired or invalid token.

Written by the indexing model from the issue text.