Add lock mechanism to prevent multiple task executions

[mcweba]

When multiple instances of vertx-redisques are deployed and periodic tasks are scheduled, the tasks will be executed in every instance.

Example from PeriodicMetricsCollector class:

private void updateActiveQueuesCount(Runnable onPeriodicDone) {
    vertx.eventBus().request(redisquesAddress, buildGetQueuesCountOperation(), (Handler<AsyncResult<Message<JsonObject>>>) reply -> {
        if(reply.failed()) {
            log.warn("TODO error handling", reply.cause());
        } else if (reply.succeeded() && OK.equals(reply.result().body().getString(STATUS))) {
            activeQueuesCount.set(reply.result().body().getLong(VALUE));
        } else {
            log.warn("Error gathering count of active queues. Cause: {}", reply.result().body().getString(MESSAGE));
        }
        onPeriodicDone.run();
    });
}

Using the EventBus, the count of active queues is retrieved and stored in the counter. However, this task is executed by every instance concurrently. This is not very efficient, because counting queues takes some time and the result will most likely be the same for all the executions.

In swisspost/gateleen there is a class called LockUtil which provides a simple lock mechanism based on Redis to prevent concurrent executions of tasks.

This feature could also be introduced in vertx-redisques to improve handling of tasks.