[Memory Engine] MemTablet creation and compatibility handling in BE

[decster]

After adding meta, BE now can create MemTablet, and put it into TabletManager, but MemTablet is not compatible with Tablet, a lot of code may break. I propose the following changes:

Step 1: when TabletManager::get_tablet is called, only return Tablet, if the underlying tablet is MemTablet, return an error, this keeps the initial code changes small.

Step 2: For methods/functionalities Tablet&MemTablet both have, refactor/extract them to BaseTablet, and change the usage to TabletManager::get_base_tablet, and it will gradually remove errors introduced by step1, this is a long going process.