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.

private final Invoker<T> originInvoker;
        private Exporter<T> exporter;
        private URL subscribeUrl;
        private URL registerUrl;
        public ExporterChangeableWrapper(Exporter<T> exporter, Invoker<T> originInvoker) {
            this.exporter = exporter;
            this.originInvoker = originInvoker;
        }
        public Invoker<T> getOriginInvoker() {
            return originInvoker;
        }
        @Override
        public Invoker<T> getInvoker() {
            return exporter.getInvoker();
        }
        public void setExporter(Exporter<T> exporter) {
            this.exporter = exporter;
        }
        @Override
        public void unexport() {
            String key = getCacheKey(this.originInvoker);
            bounds.remove(key);
            Registry registry = RegistryProtocol.INSTANCE.getRegistry(originInvoker);
            try {
                registry.unregister(registerUrl);
            } catch (Throwable t) {
                logger.warn(t.getMessage(), t);
            }
            try {
                NotifyListener listener = RegistryProtocol.INSTANCE.overrideListeners.remove(subscribeUrl);