Implement batch-processing of nodes

[jkva]

Since this process can take a long time on larger setups, this should be a happening asynchronously and track its progress such that it can be stopped and resumed later.

[jkva]

Effectively, the module user should be:

• able to see the current status of a running batch job (if any)
• able to stop a running batch job (if any)
• able to start a new batch job.

This should be implemented using asynchronous updates.

[jkva]

A batch job is defined as a process in which Drupal nodes are processed for images using a configurable (tunable) batch size, and images are submitted as a createResourcesRequest using the API client.