add common/config and common/global

Now we only have one option: client_type

Signed-off-by: Dong Yuan <yuandong1222@gmail.com>
add some common structs

ClusterMap
Context
Global

Signed-off-by: Dong Yuan <yuandong1222@gmail.com>
finish librados::rados_create

Signed-off-by: Dong Yuan <yuandong1222@gmail.com>