@Override
        public Database create(Params params) {
            return new FakeZooKeeperDatabase(context, params.listener);
        }
Memory-backed fake DB implementation that tries to mirror the semantics of the
(synchronous) ZooKeeper DB implementation. By itself this fake acts as if a quorum
with a _single_, local ZK instance has been configured. This DB instance cannot be
used across multiple cluster controller instances.

Threading note: we expect all invocations on this instance to happen from the
main cluster controller thread (i.e. "as-if" single threaded), but we wrap everything
in a mutex to stay on the safe side since this isn't explicitly documented as
part of the API,
clustercontroller-core/src/test/java/com/yahoo/vespa/clustercontroller/core/FakeZooKeeperDatabase.java