BUG-650: cache translated future

We are instantiating a done future with fixed result -- this is an
invariant, which can readily be shared. do precisely that by retaining a
single global instance.

Change-Id: I0a24adf6f3df4794fda5148c04baed7bbde11c64
Signed-off-by: Robert Varga <rovarga@cisco.com>
Merge "BUG-650: cache translated future"