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
I could do
`tailSize = numGCRuns - mostRecentGC`
`tail = pauseBufSize - tailSize`