nsqd: panic when nsqd exits

[wangcn]

How to reproduce?

1. Pull master branch. The lastest commit is c164c77
2. Make
3. Start nsqd
4. Kill nsqd

It panics and outputs "panic: close of closed channel".

#1319 (comment)
ad73d05#diff-4fb927abdfbf2db38da702b7131f128fa6d7e4cbd3d6d7baa3f4271eb2949956R471
These lines cause nsqd.Exit to be called twice. We can just simply use go-svc to handle all signals.

[mreiferson]

ping @jehiah

[mreiferson]

This will be fixed in #1331