No fail on start commands

```
var redis = require('redis-sentinel-client');

var client = redis.createClient(26379, '127.0.0.1');
client.set("foo", "bar");
setTimeout(function(){
  client.get('foo', function(err, reply){
    console.log(reply);
  })}, 500);
```

This will cause an error:
```
redis-sentinel-client/index.js:292
  return client.send_command.apply(client, arguments);
               ^
TypeError: Cannot read property 'send_command' of undefined
```

This PR does two things.
1) don't error if we try to send a command to a brand new redis sentinel connection
2) re-enable the disable_flush functionality that got lost when we went back to an upstream release of node_redis.
 
I opened an upstream pr for my fork https://github.com/mranney/node_redis/pull/675