Rest-Rpc adds a uncommon class ServiceClassHolder

[Jeff-Lv]

As the class ServiceClassHolder's javadoc saying,
“TODO this is just a workaround for rest protocol, and now we just ensure it works in the most common dubbo usages”

We need to refactor it and remove this temp solution with a common way.

Expected Result

Impove the way of rest impletation

Actual Result

no import the special class for REST to the common module of Dubbo