Do not use ExecutorService unnecessarily

Executor is the interface we need, not ExecutorService. ExecutorServices
are more complex, and we do not care about shutting them down anyway.

So require only an Executor, letting callers to deal with its lifecycle,
which allows us to use more efficient MoreExecutors.directExecutor()
instead.

Change-Id: Ide57266cc1fac395e545b466bf3b5ad86dee7b2f
Signed-off-by: Robert Varga <rovarga@cisco.com>
(cherry picked from commit 195ee42d6fe586b331ec949a0f3579a27bb51eed)