Introduce new example and helper classes to tell CP users how to do filtering on scanners

Finally we decided that CP users should not have the ability to create StoreScanner or StoreFileScanner, so it is impossible for them to filter out some cells when flush or compaction by simply provide a filter when constructing StoreScanner.

But I think filtering out some cells is a very important usage for CP users, so we need to provide the ability in another way. Theoretically it can be done with wrapping an InternalScanner, but I think we need to give an example, or even some helper classes to help CP users.