Add a new KV wrapper GenericObjectMap

Now we already have DBObjectMap which implement ObjectMap and other
interfaces, and ObjectMap.h implied that ObjectMap is used to encapsulates
the FileStore key value store. There exists limitation in current DBObjectMap
implementation, such as lacking of "coll_t" in "key", complicated prefix
hard-coded and inflexible extending.

So in order to provide a more flexible API and clear implementation to wrap KV
store, I copy the origin DBObjectMap and redesign the partial implementation.
Adding "coll_t" argument to all API and export "prefix" to callers. Prefixes
are divided into two parts "INTERN" and "USER". "INTERN" keys used by self to
manage and "USER" keys are managed by callers. Besides above, misc fixes are
imported such as more clear member function name and extendible header
structure.

Signed-off-by: Haomai Wang <haomaiwang@gmail.com>
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>
Make ObjectStore unit tests shared with KeyValueStore

Via gtest's value-parameterized feature, let the successors of
ObjectStore share the same unit tests.

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