cluster: wait for PD finish loading regions after PD restart or upgrade

[djshow832]

What problem does this PR solve?

close #2461

What is changed and how it works?

When checking PD health, also check pd/api/v2/ready to see if regions are loaded.
This applies to both upgrade and reload.

Check List

Tests

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

Code changes

• Has exported function/method change
• Has exported variable/fields change
• Has interface methods change
• Has persistent data change

Side effects

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

Related changes

• Need to cherry-pick to the release branch
• Need to update the documentation

Release notes:

- Wait for PD finish loading regions after PD restart or upgrade

[ti-chi-bot]

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[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 25 lines in your changes missing coverage. Please review.

Project coverage is 45.62%. Comparing base (e8c63c6) to head (9151b59).

❗ Current head 9151b59 differs from pull request most recent head 3ee0646

Please upload reports for the commit 3ee0646 to get more accurate results.

Files with missing lines	Patch %	Lines
pkg/cluster/api/pdapi.go	0.00%	25 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2521      +/-   ##
==========================================
- Coverage   45.65%   45.62%   -0.03%     
==========================================
  Files         337      337              
  Lines       36436    36460      +24     
==========================================
- Hits        16633    16632       -1     
- Misses      17774    17799      +25     
  Partials     2029     2029              

Flag	Coverage Δ
tiup	33.81% <ø> (ø)
unittest	21.61% <0.00%> (-0.02%)	⬇️

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