Worker: use `log_batch` event for batch logs

[josephjclark]

Currently the worker batch log implementation uses the good old run:log event and just adds an array of logs.

@midigofrank suggests it'll be cleaner to use a different event with a different structure, rather than having some polymorphous thing.

So let's edit the batch implementation to put out a different event. Maybe run:log_batch, because run:logs is a bit subtle.

We'll keep the feature flag on the worker side

[josephjclark]

false alarm