Load region info from pd in batch

[SeaRise]

Enhancement

When a table is very large, the number of regions can be significant. Additionally, because TiDB loads region information from PD with single concurrency and one region per request, loading the region information of a large table can be very slow.

related #14320

[SeaRise]

/assign @SeaRise

[SeaRise]

/label affects-7.5

[SeaRise]

/label affects-7.1

[SeaRise]

/label bug

[ti-chi-bot]

@SeaRise: The label(s) bug cannot be applied. These labels are supported: fuzz/sqlancer, challenge-program, compatibility-breaker, first-time-contributor, contribution, good first issue, correctness, duplicate, proposal, security, ok-to-test, needs-ok-to-test, needs-more-info, needs-cherry-pick-release-5.4, needs-cherry-pick-release-6.1, needs-cherry-pick-release-6.5, needs-cherry-pick-release-7.1, needs-cherry-pick-release-7.5, needs-cherry-pick-release-7.6, affects-5.4, affects-6.1, affects-6.5, affects-7.1, affects-7.5, affects-7.6, may-affects-5.4, may-affects-6.1, may-affects-6.5, may-affects-7.1, may-affects-7.5, may-affects-7.6.

In response to this:

/label bug

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[SeaRise]

/type bug

[SeaRise]

/untype enhancement

[SeaRise]

/severity minor