nsqd: cleanup IDENTIFY "id" options

In #296 we added `user_agent` to `IDENTIFY`. We now have 3 somewhat fuzzy options that need consolidation and clarification (`short_id`, `long_id`, `user_agent`).

I propose:

1. keep `user_agent` - it is intended to identify the *agent* (client library)
2. change `long_id` to `hostname` - it is now crystal clear what it's supposed to describe
3. change `short_id` to `client_id` - this is expected to identify the consumer, i.e. `queuereader_realtime-5` (IMO this does not need to be globally unique as `id` + `hostname` can serve that purpose externally, in `nsqadmin` for example)

We can do this in a way that is backwards compatible, of course.  For example if `client_id` is not sent we can continue to default to the "short" version of `hostname`.  Also, client libraries can continue to send `short_id` and `long_id` for some transition period.

thoughts @jehiah?