bug: gc memory collection

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
looks like in production, nsqd is only GC'ing ~30/minute, so I think its worth keeping track of these stats accurately, its well within the range of being useful.