Install in-memory store in tsmon by default.
Install in-memory store in tsmon by default.

It allows unit tests that do not care about tsmon (and thus do not initialize
it) to run (and not crash with nil dereference trying to call Store().Incr()).

TBR=dsansome@chromium.org
BUG=570879

Review URL: https://codereview.chromium.org/1618903002

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go