Use fibers in BlockRepository::getVisibleBlocksPerRegion()

Problem/Motivation

As identified in #1237636: Lazy load multiple entities at a time using fibers, there is potential to bulk load more entities by running block access checks in fibers.

Likely want to wait on revolt, but creating this not lose track of this.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet