Refactor KVStore related

[wenhaocs]

Introduction

We need to prepare for stores other than NebulaStore, e.g., cloud storage. In this case, NebulaStore can be some kind of KVStore with Raft. KVStore should be the general interface. However, the current KVStore has some logic related to Raft. In addition, NebulaStore inherit both KVStore and Handler, and has its own member function as well, which is not clear. What we want to achieve is
(1) A generic KVStore, which supports both cloud and premise underlying storage.
(2) NebulaStore is a member of CachedKVStore, which implements something else, e.g., RaftKVStore.
(3) All up layers in storage call KVStore interface