Add a new KV wrapper GenericObjectMap

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>