Adding a new storage engine, we need to make an extensible tablet interface, so olap/StorageEngine can support and manage new tablet types. To start, this commit creates a class BaseTablet and make Tablet and new MemTablet inherit this base class, some common fields & methods are moved to BaseTablet class, which fields and methods belong to base/old class is not finalized yet, it will change as the project evolves. Fix #3384