Test memcached integration

[aslo]

Verify that an application can read/write from memcached.

https://cloud.google.com/appengine/docs/flexible/java/caching-application-data

[aslo]

see also GoogleCloudPlatform/jetty-runtime#15

[meltsufin]

@aslo Does this still make sense to test?

[aslo]

Seeing as how Memcache isn't currently available in the Flex environment, I don't think this really makes sense at the moment. Close this?

[meltsufin]

It will be available at some point. So, let's leave it open.