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
I didn't really have a goal of it returning only the GC pauses between calls to stats, because it is theoretically possible to wait so long that you miss ones, too.  I was just trying to eliminate returning 0s.

Why don't we just change this to `len(memStats.NumGC) <= 256` and be done with it?