backup: check the store state by last heartbeat (#43099)

[ti-chi-bot]

This is an automated cherry-pick of #43099

What problem does this PR solve?

Issue Number: close #42973

Problem Summary:
The current implementation cannot detect whether a store is down. So it keeps retry sending request to an offline store.

What is changed and how it works?

This PR checks the LastHeartbeatTS of the store and give up to request the store once the gap is greater than 20s (also defined in the PD package.)

Check List

Tests

• Manual test (add detailed scripts or steps below)
  Backup over a cluster with one node down, finished in reasonable time.

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

Fixed a bug that caused backup get very slow when there are down TiKV nodes in the cluster.

[ti-chi-bot]

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