Polish Dubbo 2.6.x adapter and unify callback registry into DubboAdap…

…terGlobalConfig (alibaba#1572)

* Unify Dubbo callback registry (for fallback and origin parser) into DubboAdapterGlobalConfig
* Polish default fallback implementation (wrap exception with RpcResult rather than directly throw it out)

Signed-off-by: Eric Zhao <sczyh16@gmail.com>

sentinel-adapter/sentinel-dubbo-adapter/README.md

@@ -69,10 +69,4 @@ flow control, degrade or system load protection. You can implement your own `Dub
 and then register to `DubboAdapterGlobalConfig`.
 If no fallback is configured, Sentinel will wrap the `BlockException` as the fallback result.
 
-Besides, we can also leverage [Dubbo mock mechanism](http://dubbo.apache.org/en-us/docs/user/demos/local-mock.html) to provide fallback implementation of degraded Dubbo services.
-
-## Global dubbo provider origin parse
-
-Sentinel Dubbo Adapter supports global origin parse for provider.
-You can implement your own `DubboOriginParser` interface
-and then register to `DubboOriginParserRegistry`. If no originParse is configured, Sentinel will user dubbo url property application.
+Besides, we can also leverage [Dubbo mock mechanism](http://dubbo.apache.org/en-us/docs/user/demos/local-mock.html) to provide fallback implementation of degraded Dubbo services.