journal_server: don't write to tlfJournals in enableLocked
journal_server: don't write to tlfJournals in enableLocked

Since now it might be called in parallel by EnableExistingJournals,
this would cause a concurrent read/write map access failure.  Instead,
return the tlfJournal and have the caller add it to the map under the
lock.

Issue: KBFS-2065

keybase/client,keybase/client,keybase/client,keybase/kbfs,keybase/client,keybase/client,keybase/client,keybase/kbfs,keybase/client,keybase/client,keybase/kbfs,keybase/client,keybase/client,keybase/client,keybase/kbfs,keybase/client