VID-2082 VideoHandlerController::getVideoList cache in memc and varnish

RFCR @garthwebb @macbre 

This method is being called a lot. Let's cache it to reduce the stress it imposes on database and backend servers.

https://wikia-inc.atlassian.net/browse/VID-2082
I see.  Oh PHP, only you would fix a serious bug, like a memory leak, and "fix" it by putting a bold warning in the documentation about how to code around it.