Add `control?` support to memory adapter

it can just be defined as !present? or some other - the main point is that it should be defined, so that the memory adapter's API matches the base adapter

After a discussion, it seems that control is meant to mean the opposite of present.

I think we should rename the method to be more clear, and just use control under the hood for the split adapter.

[matthewjf]

In split, the control treatment is returned if the flag is not present or an error occurs. It's a subtle distinction from !present:

https://help.split.io/hc/en-us/articles/360020528072-Control-treatment