acme/autocert: fix racy tests

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>
2 files changed