remove BatchJob#full?

A BatchJob can't be appended to
remove batch size control

It's not clear how this will work with backends, so just remove it for
now.
make batch processing a module

Instead of a Qu::BatchJob subclass, batch processing is now a
Qu::Job::BatchProcessing module. Include the module to get batch
processing primitives.
add PayloadBatch collection class

A PayloadBatch is a collection of payloads intended to be created by a
backend supporting batch pop.

For now, PayloadBatch executes #perform on all payloads in its batch.