system-variable: add tidb_analyze_distsql_scan_concurrency

[hawkingrei]

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2023-12-27 01:31:25.379233479 +0000 UTC m=+1615776.416460406: ☑️ agreed by hfxsd.
• 2023-12-28 09:52:24.578063456 +0000 UTC m=+1732235.615290382: ☑️ agreed by ran-huang.

[hfxsd]

/approve

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