* Codebase for zipkin span persistence.

* Fix missing fields in storage.

* Miss the latency field.

* Finish some tests.

* Fix wrong latency.

* Finish doc and reset application.yml

* Make the description more clear.

* Change some codes to make zipkin receiver works, not done yet.

* @adriancole Zipkin receiver is coming back again.

* Fix document and fix for Zipkin receiver.

* Fix some bugs.

* Fix register bug.

* Fix transfer bug for new backend core.