Improve the performance of the HTTP API for getting all regions (#1970)

[rleungx]

What problem does this PR solve?

cherry-pick #1970 to release-3.1

What is changed and how it works?

Optimize convertToAPIRegions and h.rd.JSON

Check List

Tests

• Unit test

Related changes

• Need to be included in the release notes

[rleungx]

/run-all-tests

[rleungx]

/run-all-tests

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (release-3.1@7a83ca7). Click here to learn what that means.
The diff coverage is 100%.

@@              Coverage Diff               @@
##             release-3.1    #1987   +/-   ##
==============================================
  Coverage               ?   69.47%           
==============================================
  Files                  ?      171           
  Lines                  ?    16807           
  Branches               ?        0           
==============================================
  Hits                   ?    11677           
  Misses                 ?     4143           
  Partials               ?      987

Impacted Files	Coverage Δ
server/core/region.go	93.39% <100%> (ø)
server/api/router.go	100% <100%> (ø)
server/api/region.go	66.8% <100%> (ø)

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[lhy1024]

/merge

[sre-bot]

/run-all-tests