* Improve the implementation mechanism of queue mode. #96

* refactor the code .

* add Copyright & License description into header text of code files.

* refactor

* set version to 2.2.0

* update unit tests.

* add exception class to process the publish send and subscriber exectution exception

* modify first retry time to three.

* code refactor.

* add retry mechanism

* code refactor

* refactor consumer execution

* fix spell error

* remove dashboard  `processing ` content.

* Modify the retry to retry the message only 4 minutes ago.

* update samples

* update ci configuration.

Because of deadlock and performance issues, mysql does not use transactions when consuming messages, and requeue the message to queue table when they exception. (#36,#68)