During install/upgrade, caching is disabled. This change provides a
way to temporarily enable caching (using in-memory cache storage only)
within a specific function; caches are deleted afterwards.
Adding this to two locations improves install performance quite a lot.
Caching is not enabled during the parts of plugin installation that
can vary for individual plugins (install.php/upgrade.php) as these
might be relying on its absence, for example by making direct database
changes.
sam marshall