client-go/safeTS: reduce GetStoreMinResolvedTS call

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Bug Report

Please answer these questions before submitting your issue. Thanks!

related to tikv/client-go#922

currently, we try to GetStoreMinResolvedTS for all stores from PD every 2 seconds

Background

For txn_scope

txn_scope will be global in major situation, So we can judge @@txn_scope value which from the config:
when find is global, we can get cluster-level resolved ts to avoid using kv grpc.

If not, we need to enhance min resolved ts usage:

For min resolved ts

Nowadays we have 2 api interface for obtaining min resolved ts

• /pd/api/v1/min-resolved-ts: obtain cluster's min resolved ts
• /pd/api/v1/min-resolved-ts/{store_id}: obtain each store's min resolved ts

We can use the approach I to get cluster's min resolved ts when @@txn_scope is global.
For client-go's updateSafeTS, we call approach II for each store, which is not necessary.
We can extend the approach I params to obtain specified store via a list to reduce the cost per call after api: Extend the min-resolved-ts api to support getting the specified store pd#6880 merged

1. Minimal reproduce step (Required)

For example when we have 100 tikvs and 100 tidbs, then the connections for pd/api/v1/min-resolved-ts to PD would be 100*100/2=5000 in 1 seconds.

2. What did you expect to see? (Required)

reduce pd/api/v1/min-resolved-ts call

3. What did you see instead (Required)

For example when we have 100 tikvs and 100 tidbs, then the connections for pd/api/v1/min-resolved-ts to PD would be 100*100/2=5000 in 1 seconds.

4. What is your TiDB version? (Required)

master

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/affect-7.1

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release-6.5 has been fixed in #46143 (comment)