* The prototype of topN sql worker.
* Add scope and new manual dispatcher.
* no message
* Finish topN persistence codes. Not test yet. And query have not added.
* Finish the top n database statement persistent.
* Support different slow thresholds for different db types.
* Fix wrong db default threshold
* Finish new query protocol binding.
* Provide query empty implementation and sync ui.
* Finish all codes, hope it works :P
I will run the tests tonight.
* Fix
* Fix
* Fix a startup issue.
* Add time bucket to persistent.
* Fix wrong query result column name.
* Fix Database query.
* Fix checkstyle.
* Fix wrong order logic, and add a test case to verify, to fix https://github.com/apache/incubator-skywalking/pull/2239#discussion_r255948249
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