memCache used an unsynchronized map, which failed go test -race. Add a mutex and constructor function to fix it. Change-Id: Iddaa492ea1e3d7747965c6423368baa6556402ed Reviewed-on: https://go-review.googlesource.com/36545 Run-TryBot: Heschi Kreinick <heschi@google.com> Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com> TryBot-Result: Gobot Gobot <gobot@golang.org>