log: more locking
log: more locking

This didn't actually cause a bug, but looks wrong.

There was a lock but there was more shared mutable state not
guarded by it.

R=golang-dev, dsymonds, r
CC=golang-dev
http://codereview.appspot.com/4760047
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go