refine delete resource group logic

[guo-shaoge]

What problem does this PR solve?

Issue Number: Close #7147

First remove meta info of resource group from memory then remove it from etcd storage.

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.

What is changed and how does it work?

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.

[ti-chi-bot]

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• disksing

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[guo-shaoge]

#7191