add redis sentinel support and custom shard key generate function

```
[
    {'name': 'server1', 'host': '127.0.0.1', 'port': 10000, 'db': 0},
    {'name': 'server2', 'host': '127.0.0.1', 'port': 11000, 'db': 0},
    {'name': 'server3', 'host': '127.0.0.1', 'port': 12000, 'db': 0},
]
```