Remove the background job handler dependency from the batch handler

[ChaseWiseman]

Right now our SV_WP_Job_Batch_Handler utility relies on the background/async job handler by simply processing chunks of a background job in batches. This works very well for plugins that support both types of job processing and those with a UI for listing ongoing jobs (so batches can be resumed, paused, etc...), but it can be overkill for plugins that don't really need background processing at all.

We should make the underlying job handler optional so that plugins don't have to worry about any of that and can just process batches until the user navigates away.