Config center optimization

[chickenlj]

What is the purpose of the change

Introduce ConfigManager, a centralized place where we can store global and shared Config instances. With ConfigManager, we can solve:

1. Duplicate Config check which we have lost in the previous version (only work in Spring)
2. Make it possible for a Builder pattern, see the comments I leave in ConfigManager.java

Brief changelog

1. Add ConfigManager
2. Related changes in several Config classes.

Verifying this change

Run samples.

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[chickenlj]

I don't know why some file changes are not correctly shown, I merged the master branch into my personal branch and this PR just showed these changes.

[carryxyh]

@chickenlj
My flow is:
master rebase upstream master
feature branch(like ConfigCenter-optimization) rebase master
push -f to force override feature branch (push -f is very important)

Is this helpful ?

[chickenlj]

Thanks carryxyh, I tried but no help.