runtime/debug: don't STW GC on SetGCPercent

As of Go 1.8, `runtime/debug.SetGCPercent` forces a full STW GC. This is an impediment to responding to memory pressure/overload.

Assuming we're not already over the new GOGC value, it should instead adjust the GC pacing parameters without necessarily triggering a GC. This is complicated by the fact that we may be mid-sweep or mid-mark, so we have to account for current progress.
