keyspace: update keyspace config TSOKeyspaceGroupIDKey during the split

[JmPotato]

What problem does this PR solve?

Issue Number: ref #6232.

What is changed and how does it work?

Update keyspace config `TSOKeyspaceGroupIDKey` during the split.

Check List

Tests

• Unit test
• Integration test

Release note

None.

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[binshi-bing]

SaveKeyspaceMeta() needs to be called in batches too, otherwise the len(keyspaces) in a split needs to be <= maxTxnOps which isn't practical even in dev with tens thousands of keyspaces. Adding batching in another place increases complexity again.

Let's discuss about it today. @rleungx

[JmPotato]

Close since the flag TSOKeyspaceGroupIDKey will be ignored for now after the discussion.