Further defer batch creation

[bensheldon]

Thinking about #912, and looking at the Batch code, I think that it would be possible to defer creating the batch.

For example, if no jobs are added to a batch, don't save it yet:

good_job/app/models/good_job/batch.rb

Line 112 in 5280b37

def add(active_jobs = nil, &block)

Further, I wonder if add should not actually enqueue the jobs, and instead the batch would store the buffer of unpersisted jobs until enqueue was called. This feels like a breaking change... but I also think it's a necessary one (I'm not sure of the use-case why someone would want to add jobs to a batch but not have them trigger an enqueue callback). This also makes the Batch more idempotent (the whole enqueue step maybe could be done in a transaction, for example)