Support 'batch' event in laconia-batch

[ceilfors]

For us to be able to process items in laconia-batch, we can listen to 'item' events. These events only contain individual items. Whenever we have a use case to process item in 'batch', we are going to have to keep an array by ourselves when 'item' events are fired and process them in batch.

Currently supported:

.on('item', (laconiaContext, item) => ... )

Proposed:

.on('batch', (laconiaContext, items) => ... )

[hugosenari]

How many items should we process at same time? :thinking: