Allow queuing in restore pool

[nickitat]

Changelog category (leave one):

• Bug Fix (user-visible misbehavior in an official stable release)

Changelog entry (a user-readable short description of the changes that goes to CHANGELOG.md):

Fix deadlock during restore database execution if restore_threads was set to 1.

[robot-clickhouse-ci-2]

This is an automated comment for commit 8be4a3d with description of existing statuses. It's updated for the latest CI running

✅ Click here to open a full report in a separate page

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[alexey-milovidov]

This is good, but why "not for changelog" and how would you explain this change?