Replace Flag Counts tables with tagged cache calls

Problem/Motivation

Direct database calls are strongly discouraged in Drupal 8. They limit backend swapability, and do not necessarily grant performance advantages sought. We use direct database calls extensively in the flag counts API, since a config or content entity was deemed Not The Right Thing.

Proposed resolution

Replace calls to the database with the Drupal 8 Cache API using tags.

Examples here: https://api.drupal.org/api/drupal/core%21modules%21system%21core.api.php...

Remaining tasks

Create patch.

User interface changes

None.

API changes

The API should remain unchanged, but no table will be available.