Support concurrent loading of Config for different namespaces (#29)

[zth9]

What's the purpose of this PR

fix #29

Which issue(s) this PR fixes:

Fixes #29

Brief changelog

• Support concurrent loading of Config for different namespaces.

Follow this checklist to help us incorporate your contribution quickly and easily:

• Read the Contributing Guide before making this pull request.
• Write a pull request description that is detailed enough to understand what the pull request does, how, and why.
• Write necessary unit tests to verify the code.
• Run mvn clean test to make sure this pull request doesn't break anything.
• Update the CHANGES log.

[github-actions]

CLA Assistant Lite bot All contributors have signed the CLA ✍️ ✅

[codecov]

Codecov Report

Merging #31 (bff4bcf) into main (265fe49) will decrease coverage by 0.08%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##               main      #31      +/-   ##
============================================
- Coverage     68.32%   68.25%   -0.08%     
+ Complexity     1191     1190       -1     
============================================
  Files           169      169              
  Lines          5156     5160       +4     
  Branches        561      561              
============================================
- Hits           3523     3522       -1     
- Misses         1369     1371       +2     
- Partials        264      267       +3     

Files Changed	Coverage Δ
...amework/apollo/internals/DefaultConfigManager.java	93.33% <100.00%> (+1.02%)	⬆️

... and 3 files with indirect coverage changes

[zth9]

I have read the CLA Document and I hereby sign the CLA

[nobodyiam]

I think the changes are looking great! Could you also please make sure to update the CHANGES.md?

[zth9]

I think the changes are looking great! Could you also please make sure to update the CHANGES.md?

All tasks have been completed.🤝

[nobodyiam]

LGTM