When Cache manually disabled, Upgrade hangs

[technosailor]

I manually disabled the WP cache long time ago (shortly after 2.0 release). When upgrading to a nightly with CACHE_DISABLED set to true, upgrade hangs on both the first and second wp_cache_flush();

Here's a patch that determins if CACHE_DISABLED is true and runs appropriate code.

wp_cache_flush() should return immediately if the cache is disabled. If that's not working, we need to fix it in the cache, not in upgrade.php, because we will break our encapsulation of the cache.

(In [3465]) Cache flush improvements from Owen. fixes #2223 #2278