codereview: speed upload
codereview: speed upload

Cuts time to upload trivial 160-file CL by 5x,
from 250 seconds to 50 seconds.

R=r
CC=golang-dev
http://codereview.appspot.com/1991047

tav/go,tav/go,tav/go,tav/go,tav/go,tav/go,tav/go