ZF-6468: "BlackHole" cache backend
Sometimes developers want temporary disable caching. The easiest way is switch backend to some other. I propose include new backend with name "BlackHole". It is always return true for storing data, false when loading/test data, and empty array for searching by tags, etc... It implement Extended interface for more compatibility.
NB: 'caching=false' option is not equal this backend, i.e. you can use this backend as part of Two_Levels backends.