md_util: decrypt the private MD into a copy after it's cached (#50)
md_util: decrypt the private MD into a copy after it's cached (#50)

Otherwise, we can have a race with multiple goroutines trying to
decrypt into the same MD concurrently.

Issue: KBFS-1161
keybase/client,keybase/client,keybase/client,keybase/kbfs,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/kbfs,keybase/client,keybase/kbfs,keybase/kbfs,keybase/client,keybase/client,keybase/client