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 List<PoolingHttpClientConnectionManager> connectionManagers = Collections.synchronizedList(new LinkedList<>());
        public void addConnectionManager(PoolingHttpClientConnectionManager connectionManager) {
            connectionManagers.add(connectionManager);
        }
        @Override
        public void run() {
            try {
                while (!shutdown) {
                    synchronized (this) {
                        wait(HTTPCLIENTCONNECTIONMANAGER_CLOSEWAITTIME_MS);
                        for (PoolingHttpClientConnectionManager connectionManager : connectionManagers) {
                            connectionManager.closeExpiredConnections();
                            connectionManager.closeIdleConnections(HTTPCLIENTCONNECTIONMANAGER_CLOSEIDLETIME_S, TimeUnit.SECONDS);
                        }
                    }
                }
            } catch (InterruptedException ex) {
                shutdown();
            }
        }
        public void shutdown() {
            shutdown = true;
            connectionManagers.clear();
            synchronized (this) {
                notifyAll();
            }
        }
    }
}