InternalConnectorRegistry.java uses non-daemon threads

[kgala2]

For InternalConnectorRegistry, it uses non-daemon threads which prevents normal shutdown of our application.
It seems like even error on the main thread does not terminate the process now with the connector, so we have batch jobs that fail and linger in this zombie state.

In order to create daemon threads, creation of executors should be changed to something like

this.executor = MoreExecutors.listeningDecorator(
    Executors.newScheduledThreadPool(8, r -> {
        Thread t = new Thread(r);
        t.setDaemon(true);
        return t;
    })
);

#2118
similar issue: GoogleCloudPlatform/alloydb-java-connector/issues/617