Use APCu caching of composer

Should potentially shave of a few ms when loading classes of apps that
don't ship their own autoloader.

Signed-off-by: Roeland Jago Douma <roeland@famdouma.nl>

lib/base.php

@@ -891,6 +891,8 @@ class OC {
 				self::$loader->setMemoryCache($memcacheFactory->createLocal('Autoloader'));
 			} catch (\Exception $ex) {
 			}
+
+			self::$composerAutoloader->setApcuPrefix($instanceId . '-mainComposer');
 		}
 	}