rest: use interface{} for REST response

Then rest.go can use an appropriate format. Likelly we support only
JSON though.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
server: support /neighbors URL

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
rest: kill RestResponse interface

We can use RestResponse struct for all request types.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>