Datastore: Reuse successful future

ImmediateFuture does not keep any state, so we can reuse a single
instance, preventing garbage buildup just a tiny bit.

Change-Id: I200662a666f4529ea82a2ac3948be9536ca32281
Signed-off-by: Robert Varga <rovarga@cisco.com>
Merge "Datastore: Reuse successful future"