[Discussion] There are so much threadpool, can we merge some threadpool into one threadpool ?

There are so much threadpool that manage only one thread , and this thread is idle. The threadpool as follow:
* org.apache.dubbo.metadata.support.AbstractMetadataReport#reportCacheExecutor for asyn write metadata.
* The HashedWheelTimer is used in Registry retry task.
* Configcenter listen the config changed, and the callback will excuted by a dependency Executor.
* ....

There are so much situation that we will use threadpool. I think can we merge this threadpool to make the threadpool is used by much more situation.

Also we should move some timer task to HashedWheelTimer?