Memory leak when exporting and importing from S3

[jepett0]

LeakSanitizer report

steps to reproduce:

1. build ydbd with --sanitize=leak flag
2. start ydbd (you can use start.sh script from the quickstart with a custom ydbd binary)
3. create table_to_backup table in your database
4. run ydb export s3 to backup the database to some S3 storage (you can use Yandex Cloud, it is free up to 1GB a month)
5. delete table_to_backup table in the database
6. run ydb import s3 to restore the database from the backup created at step 4
7. run stop.sh script to shutdown the cluster
8. observe stderr for the LeakSanitizer report