librados: fix IoCtx::from_rados_ioctx_t

librados: fix IoCtx::from_rados_ioctx_t

IoCtx::from_rados_ioctx_t creates an IoCtx out of a rados_ioctx_t.
However, this IoCtx must share ownership of the IoCtxImpl pointer with
the C API user who first called rados_ioctx_create. This must be done
via a reference count inside the IoCtxImpl.

Also add a copy constructor and assignment operator to class IoCtx,
since it's now cheap to have them.

Signed-off-by: Colin McCabe <colin.mccabe@dreamhost.com>