Support for redis AUTH Command

This patch supports redis AUTH Command, but not as User Command. 
In req_forward, this patch add auth packet in Request Queue when redis and redis_auth's length is not 0.

so. nc_connection has first flag. 

and add redis_auth keyword to conf file to use redis auth.
```c
leaf:
  listen: 127.0.0.1:22121
  hash: fnv1a_64
  redis: true
  redis_auth: testpass
  distribution: ketama
  auto_eject_hosts: true
  server_retry_timeout: 2000
  server_failure_limit: 1
  servers:
   - 127.0.0.1:3001:1 server1
   - 127.0.0.1:3002:1 server2
```

#add changes
1. support user's auth command.
 -> twemproxy filters client's auth command and status.
     and, if client is not admitted, then return -ERR