placement: refine placement options check

[xhebox]

What problem does this PR solve?

Issue Number: close #28767 and #29006

Problem Summary: Fix checks to adapt the RFC requirement.

What is changed and how it works?

1. We always prefer sugar sytax if possible
2. For non-sugar options, it is allowed to set only one leader or some followers(voter). But it is not allowed to miss both options.
3. For non-sugar options, forbid set XX_CONSTRAINTS without specifying counts.
4. For sugar options, it is allowed to omit followers when *REGIONS are set, or the revert. But you can not omit all options.

Check List

Tests

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

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

None

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• lcwangchao
• morgo

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[morgo]

LGTM

tidb> create table t1 (a int) followers=4;
Query OK, 0 rows affected (0.11 sec)

tidb> create placement policy `global` primary_region='us-central1' regions='us-central1,us-west1,asia-northeast1';
Query OK, 0 rows affected (0.08 sec)

[lcwangchao]

I prefer the old implement and regard single followers=xx configuration as non-sugar options. It is easier to understand.

[xhebox]

This one is just the reverse. It is same. And I must admit sugar options, now, are really smarter than the non-sugar one. Only if you know what you are doing, can you make a better policy with non-sugar options.

BTW, followers are allowed in both options, and you can not use it to distinguish whether it is sugar or not.

[xhebox]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: 742da59

[xhebox]

/merge

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[AilinKid]

LGTM