Optimize message memory footprint in output path

This is a follow-up issue with tasks remaining from https://github.com/Graylog2/graylog2-server/pull/19982:

TODO:
- [x] Consider not memoizing serialized messages when batch size config is still count-based:
- https://github.com/Graylog2/graylog2-server/pull/19982#discussion_r1701599821:
>What do you think about skipping the serialization if output_batch_size is configured for a number of messages instead of bytes?
Otherwise, we would always serialize early and have the memory overhead, even if we don't need it.
- [ ] Consider clearing memoized serialized value as soon as it has been sent to the indexer, to make it available to GC immediately
- https://github.com/Graylog2/graylog2-server/pull/19982#discussion_r1701625862:
>I wonder if we should clear the serialized message from the cache here to avoid keeping it in memory for the lifetime of the message object.
>
>Not sure if we can do that here because the serialized message might be used by other outputs? Right now we only use it in the Elasticsearch/OpenSearch outputs.
- [x] Not related to memory footprint, but to improve the `ImmutableMessage` interface a bit, return Immutable* collections where applicable:
- https://github.com/Graylog2/graylog2-server/pull/19982#discussion_r1701601868
- [ ] (Optional, for bonus points) Try to remove the `invalidTimestampMeter` from the serialization calls. It feels like it shouldn't be there.