Register command execution points using named WP-CLI hooks

WP-CLI runs most `wp db *` commands right after `wp-config.php` is located, but before the rest of WordPress is loaded: https://github.com/wp-cli/wp-cli/blob/d9d405a4ca01ba23e80277b8cb9000857e84e0cd/php/WP_CLI/Runner.php#L875

This is a special case system that should be switched to using named WP-CLI hooks.

From https://github.com/wp-cli/db-command/issues/1#issuecomment-299742567