Halt APC functionality

Because PHP 5.5 sometimes incorrectly reports that APC is running, even
though it doesn’t *exist*, we just have to halt its use entirely, right
up front. Per #516.
Move API key cache into Memcached/Redis

The first of a handful of APC -> Memcached/Redis transitions, per #516.
Add Memcached / Redis support

Closes #263.