nsqd: v2 protocol responses/fatal errors

In order to better support the needs of robusts, this issue covers updating the various server errors such that all are fatal (server closes connections) except `FIN` and `REQ` errors.

This also simplifies the protocol "magic" handling so that we dont log cryptic big-endian forms of the 4-byte sequences.

This updates `SUB` and `IDENTIFY` to send OK responses.

See #126 for more detailed explanation as to the motivation here.

cc @bitly/nsq