Offset not persisted when no matching event handler is found

[octonato]

The CassandraReadSideHandler processes incoming events whenever an event handler is defined for an event. While processing it, the offset is saved in the same transaction.

However, offset is not saved when a handler is not found. This is not considered an error as we simply return Future.successul(Done). Basically, we skip the event.
It would be better to save the offset so this event (and potentially a long list of explicitly ignored events) won't be offered again in case of redeployment/restart.

(code fragment extracted from here)

Flow[EventStreamElement[Event]].mapAsync(parallelism = 1) { elem =>
      handlers.get(elem.event.getClass.asInstanceOf[Class[Event]]) match {
        case Some(handler) => invokeHandler(handler, elem)
        case None =>
          if (log.isDebugEnabled)
            log.debug("Unhandled event [{}]", elem.event.getClass.getName)
          Future.successful(Done)
      }
    }.withAttributes(ActorAttributes.dispatcher(dispatcher))

Alternatively we could have a fluent API similar to what was proposed in #844. In which case, the users will need to explicit declare which events they are not interested.