Handle variations of redis quit command

The primary reason for this change it to attempt to keep the client_err stat meaningful.

The "inline" quit command will produce no response and result in closing
the connection immediately.  This type of command is valid according to the redis protocol (see http://redis.io/topics/protocol) and matches phpredis behaviour (see
https://github.com/nicolasff/phpredis)

The "standard protocol" quit
command will produce an +OK response as implemented by the redis server,
however if the client doesn't read the response we'll probably still be
incrementing the client_err stat.