[Dubbo-3629] Fix ServiceConfig duplicate export produces misleading exception

[JerryChin]

What is the purpose of the change

fix #3629

First, the existing implementation by using two variables for controlling ServiceConfig exporting state is not only awkward to code, but also can theoretically lead invalid state transitions (exported and unexported both simultaneously true).

Secondly, the exported is set in doExport() method which could be called asynchronously, this makes it hard to refactor to prevent duplication exporting.

Brief changelog

the following changes are introduced:

1. refactor the two variables exported and unexported into one AtomicInteger variable exportingStage.
2. compare and set exporting state in export() and unexport() methods before doing anything.
3. remove the synchronized from the two methods.
4. add unit-test for this fix to verify everything works as expected.

The existing exporting state transition flow is as follows, remains unchanged after this PR :

        +----------------+
        | INITIAL_STAGE  |
        +----------------+
              |     ^
              |     |
              V     |
        +-----------------+
        | EXPORTING_STAGE |
        +-----------------+
                 |
                 |
                 V
        +-----------------+
        | EXPORTED_STAGE |
        +-----------------+
                 |
                 |
                 V
        +-----------------+
        | UNEXPORTED_STAGE|
        +-----------------+

Verifying this change

Necessary unit-test is added, see ServiceConfigTest

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[AlbumenJ]

@JerryChin hi, thanks for your contribution

Please merge the latest master branch to resolve conflict file

[AlbumenJ]

Close for long time no response.

Please feel free to reopen if you have any question.

If you think these changes still useful in the latest master branch, please submit a new pull request.