tools: remove RBD specific logic in immutable obj cache

tools: remove RBD specific logic in immutable obj cache

Make the daemon more generic(pool name + object id) so it
can service both RBD and RGW.
Also supports caching partial object(even empty), the caller
needs to append_zero to read buffer if read on those objects.

Signed-off-by: Yuan Zhou <yuan.zhou@intel.com>