sync: release Pool memory during second and later GCs

Pool memory was only being released during the first GC after the first Put.

Put assumes that p.local != nil means p is on the allPools list.
poolCleanup (called during each GC) removed each pool from allPools
but did not clear p.local, so each pool was cleared by exactly one GC
and then never cleared again.

This bug was introduced late in the Go 1.3 release cycle.

Fixes issue 8979.

LGTM=rsc
R=golang-codereviews, bradfitz, r, rsc
CC=golang-codereviews, khr
https://codereview.appspot.com/162980043