ddl: use kv.Key as reorg progress marker instead of kv.Handle

[tangenta]

What problem does this PR solve?

Issue Number: close #20727

Problem Summary:

For now, ADD INDEX(as well as some other DDL jobs that need data-reorganization) assume that the 'startKey' and 'endKey' of a region can always be decoded as handle out:

tidb/ddl/backfilling.go

Line 359 in 937949f

func decodeHandleRange(keyRange kv.KeyRange) (kv.Handle, kv.Handle, error) {

What is changed and how it works?

What's Changed:

The reason why TiDB decodes keys to handles during 'ADD INDEX' is that the reorganization progress is persisted in storage. The handles are used as the progress marker. This PR change the type of marker to kv.Key.

How it Works:

Using `kv.Key as the marker skips the decoding routine.

Related changes

N/A

Check List

Tests

• Unit test
• Integration test

Side effects

• Compatibility: this PR should not break compatibility because the binary representation of the progress marker is unchanged.
• Performance: to improve human readability, the key is need to be decoded every time TiDB logs.

Release note

• No release note

[AilinKid]

Rest LGTM

[AilinKid]

LGTM

[wjhuang2016]

LGTM

[tangenta]

/merge

[ti-srebot]

/run-all-tests