region_syncer: reduce saveKV of client (#3808)

[ti-chi-bot]

This is an automated cherry-pick of #3808

What problem does this PR solve?

What is changed and how it works?

• By introducing part of the heartbeat processing logic in the client of region_syncer, unnecessary saveKV operations are reduced.

Check List

Tests

• Unit test

Code changes

Side effects

Related changes

Release note

Improved the performance of synchronizing Region information between PDs.

[ti-chi-bot]

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[HunDunDM]

Sorry to be closed due to my misoperation, #3993 will replace it.