tfa.cache.memcache is not using memory cache backend

Renames the service to cache.tfa_memory as the service provider uses the name after the first period for the bin name.

As we have not released this in a tagged release we do not need to perform any BC on this change.

Realized using register() would require a significant amount of mocking.

Since this service is important for transferring information between services I have added a functional test to validate the backed is memory based.

Automatically closed - issue fixed for 2 weeks with no activity.