Currently per-sizeclass stats are lost for destroyed MCache's. This patch fixes this.
Also, only update mstats.heap_alloc on heap operations, because that's the only
stat that needs to be promptly updated. Everything else needs to be up-to-date only in ReadMemStats().

R=golang-dev, remyoudompheng, dave, iant
CC=golang-dev
https://golang.org/cl/9207047