Scan during modify scheme

[vlad-gogov]

Scan returns an error during the execution of the schema modify (SELECT COUNT(*) tableName;)
Test: https://paste.yandex-team.ru/12aa0db2-fca9-4a7b-9fd7-75fdc6b95c82
Log: https://paste.yandex-team.ru/0e899bc1-d275-4b90-81e7-e35f5f21ca13