Crash under the load

async_write in RedisClientImpl::asyncWrite causes a crash under the load. Should be wrapped by strand as well:

```
        boost::asio::async_write(socket, buffers,
                strand.wrap(std::bind(&RedisClientImpl::asyncWrite, shared_from_this(), std::placeholders::_1, std::placeholders::_2)));
```

This solves the issue.