nsqd: GC stats bugs

from http://golang.org/src/pkg/runtime/mem.go?s=342:1966#L3

`PauseNs` is *always* 256, but a circular buffer, while NumGC's will be ever increasing...

So a potential memory leak for long running nsqd's, as well as incorrectly calculating GC percentiles since the *last* statsd loop
would it be possible to write very basic tests for the few edge cases we expect, since you've refactored this to be a function?

1. length of 0 returns empty slice
2. num runs < buffer size (some data loss)
3. unwrapped
4. wrapped

My brain is having a hard time unrolling the circular buffer, and this would just make everyone feel better.

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