old/netchan: fix data race on client hashmap Fixes issue 2713.
old/netchan: fix data race on client hashmap
Fixes issue 2713.

R=golang-dev, r
CC=golang-dev, mpimenov
http://codereview.appspot.com/5545065

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