runtime: transfer whole span from MCentral to MCache

runtime: transfer whole span from MCentral to MCache
Finer-grained transfers were relevant with per-M caches,
with per-P caches they are not relevant and harmful for performance.
For few small size classes where it makes difference,
it's fine to grab the whole span (4K).

benchmark          old ns/op    new ns/op    delta
BenchmarkMalloc           42           40   -4.45%

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/9374043