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

[rleungx]

What problem does this PR solve?

cherry-pick #1970 to release-3.0

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

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (release-3.0@7a5909e). Click here to learn what that means.
The diff coverage is 100%.

@@              Coverage Diff               @@
##             release-3.0    #1986   +/-   ##
==============================================
  Coverage               ?   69.42%           
==============================================
  Files                  ?      170           
  Lines                  ?    16757           
  Branches               ?        0           
==============================================
  Hits                   ?    11634           
  Misses                 ?     4144           
  Partials               ?      979

Impacted Files	Coverage Δ
server/core/region.go	93.47% <100%> (ø)
server/api/router.go	100% <100%> (ø)
server/api/region.go	68.8% <100%> (ø)

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[nolouch]

/run-all-tests