[DNM] mcs: refine delete resource group logic to avoid corner case of tiflash

[guo-shaoge]

What problem does this PR solve?

Issue Number: Close #7147

What is changed and how does it work?

Why:

1. tiflash will call PD grpc of GetResourceGroup() to add meta info to itself. GetResourceGroup() only check meta info from memory
2. tiflash delete resource group info by watching PD etcd
3. So this can happend:
   1. User delete resource group by SQL
   2. The current implementation of DeleteResourceGroup() first delete info from etcd, so TiFlash watching will be notified, so it delete rg info.
   3. MPPTask that has already been dispatched comes to tiflash, tiflash fetch from PD using GetResourceGroup() successfully
   4. PD remove rg meta info from memory.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Code changes

• Has the configuration change
• Has HTTP API interfaces changed (Don't forget to add the declarative for the new API)
• Has persistent data change

Side effects

• Possible performance regression
• Increased code complexity
• Breaking backward compatibility

Related changes

• PR to update pingcap/docs/pingcap/docs-cn:
• PR to update pingcap/tiup:
• Need to cherry-pick to the release branch

Release note

None.

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[codecov]

Codecov Report

Merging #7191 (ed1e5c6) into master (3c632b1) will increase coverage by 0.20%.
Report is 20 commits behind head on master.
The diff coverage is 16.66%.

@@            Coverage Diff             @@
##           master    #7191      +/-   ##
==========================================
+ Coverage   74.40%   74.60%   +0.20%     
==========================================
  Files         440      442       +2     
  Lines       47180    47622     +442     
==========================================
+ Hits        35104    35529     +425     
+ Misses       8986     8980       -6     
- Partials     3090     3113      +23     

Flag	Coverage Δ
unittests	74.60% <16.66%> (+0.20%)	⬆️

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[guo-shaoge]

/hold