Use more thread pools in BACKUP/RESTORE to avoid its hanging in tests

[vitlibar]

Changelog category:

• Not for changelog

This PR is a follow-up to #55132, it adds two more thread pools to handle BACKUP ON CLUSTER ASYNC and RESTORE ON CLUSTER ASYNC without possible hanging. The PR also introduces lazily creation of such thread pools.

[robot-ch-test-poll1]

This is an automated comment for commit da5f48e with description of existing statuses. It's updated for the latest CI running

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[alesapin]

Too complex and general solution, which doesn't protect from the mentioned error. But ok, let's do it this way.

[alesapin]

Maybe better to make unordered map for simplicity?

[vitlibar]

done

[alesapin]

@vitlibar WDYT?

[vitlibar]

@vitlibar WDYT?

I rewrote my code using std::map.

[vitlibar]

Too complex and general solution, which doesn't protect from the mentioned error. But ok, let's do it this way.

Why it doesn't protect? I mean now the parallel tasks waiting each others are going to be always in different thread pools. And the reason of that deadlock was exactly that a task could wait another task from the same thread pool.