Add option to do batch processing to core's Bulk Operations.

[argiepiano]

Description of the need

Currently, core's Bulk Operations (BO) processes the requested action in one page request. This can be problematic for views with lots of results or for actions that are complex or slow. This problem will get amplified when #3798 is merged.

Proposed solution

Add the option for batch processing to core's BO. This could mirror what contrib's VBO does.

I'm thinking that the best way to handle batch processing is to mirror what VBO does. action_execute() (or action_do() in D7) do not inherently do batch process of the actions. Instead this is handled by VBO - so it should be handled by the Bulk Form views handler in Backdrop's case.

Alternatives that have been considered

None.

Draft of feature description for Press Release (1 paragraph at most)

Backdrop's Bulk Operations views handler now includes the option to do batch processing of actions ...