Add Empty.{completed,immediate}Future()

Add Empty.{completed,immediate}Future()

A number of downstream users are using Empty as a result of a future
which should only report empty success without relying on
java.lang.Void.

Since we are not playing favorites, let's expose two versions in keeping
with Java/Guava naming conventions:
- completedFuture() is the CompletionStage
- immediateFuture() is the ListenableFuture

JIRA: YANGTOOLS-1566
Change-Id: I5897ef561f562440e5d81fa7b0711e139dc6d6ee
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>