close #922

Background

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

There are 2 repair step in this pr:

1. We can use the approach I to get cluster's min resolved ts when @@txn_scope is global.
2. 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

Summary pr

step 1

judge @@txn_scope value which from the config:

• when find is global, we can get cluster-level resolved ts.
• when find is not global, we need to extend approach II.

step 2

use approach I's extension to get all stores.