Add KeyValueStore implementation

Add KeyValueStore implementation

KeyValueStore is another ObjectStore implementation with FileStore. It
uses KV store wrapper(StripObjectMap) which inherited GenericObjectMap
to implement ObjectStore APIs.

Each object has a header key in KV backend, which encapsulated the metadata
of object such as size, the status of keys. A complete object data maybe spread
around multi keys. The CRUD operation of object need to access the header key
of object to know the details, then the actual data keys will be get.

Now the actual KV backend of KeyValueStore is only LevelDB, more KV backend
(RocksDB, NVM API) will be introduced in the near future.

Signed-off-by: Haomai Wang <haomaiwang@gmail.com>