Add support for report batching in Mixer

[geeknoid]

The interface for REPORT adapters supports receiving an array of instances at a time, but Mixer currently only delivers single items. Given that Mixer receives reports in batches, it would likely be more efficient for Mixer to process an incoming batch of reports as a whole and deliver that whole batch to the adapter en masse.

We should evaluate whether this would actually be a win. There will be transient memory allocations and other overheads associated in batching in Mixer. So it might not be a good thing overall to batch in Mixer like that. It's especially interesting when considering the adapters might want to batch things in a different way than Mixer does.

[geeknoid]

See #849 for ideas on necessary config for this, and see #1501 about supporting generic per-adapter config.