Do not use JdkFutureAdapters

Do not use JdkFutureAdapters

Expose a ListenableFuture from our APIs, so that callers can hook to
async processing.

Where we are using an executor, wrap the invocation in Futures.submit(),
so we get a proper ListenableFuture.

Finally fix a thinko, where we would be reporting a success before the
enqueued task finishes.

Change-Id: Ideffb18ea6fbe47ca4d56616a52891b3155088f1
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>