When I set `path` in ServiceConfig or `contextpath` in ProtocolConfig, RestProtocol throws NPE on this line:
https://github.com/apache/incubator-dubbo/blob/master/dubbo-rpc/dubbo-rpc-rest/src/main/java/org/apache/dubbo/rpc/protocol/rest/RestProtocol.java#L93
```java
Class implClass = ApplicationModel.getProviderModel(url.getServiceKey()).getServiceInstance().getClass();
```

<dubbo:service path="test" interface="org.apache.dubbo.samples.rest.api.facade.UserRestService" ref="userRestService"
                   protocol="rest" validation="true"/>

This is because we use `group/interface:version` as the key when putting ProviderModel:
```java
Application.initProviderModel(group/interface:version, providerModel)
```
but use `group/path:version` to get, as showed above.

I think to unify to `path` would solve this problem.

registry.unsubscribe(subscribeUrl, listener);
                DynamicConfiguration.getDynamicConfiguration()
                        .removeListener(subscribeUrl.getServiceKey() + CONFIGURATORS_SUFFIX,
                                serviceConfigurationListeners.get(subscribeUrl.getServiceKey()));
            } catch (Throwable t) {
                logger.warn(t.getMessage(), t);
            }
            executor.submit(() -> {
                try {
                    int timeout = ConfigurationUtils.getServerShutdownTimeout();
                    if (timeout > 0) {
                        logger.info("Waiting " + timeout + "ms for registry to notify all consumers before unexport. " +
                                "Usually, this is called when you use dubbo API");
                        Thread.sleep(timeout);
                    }
                    exporter.unexport();
                } catch (Throwable t) {
                    logger.warn(t.getMessage(), t);
                }
            });
        }
        public void setSubscribeUrl(URL subscribeUrl) {
            this.subscribeUrl = subscribeUrl;
        }
        public void setRegisterUrl(URL registerUrl) {
            this.registerUrl = registerUrl;
        }
    }
}