favorites: add a non-blocking Add interface
favorites: add a non-blocking Add interface

Also batch adds together when a duplicate one is already in flight.

Suggested by @akalin-keybase.

Issue: #812

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