…delta files for deleted tablespaces in incremental backup directory

Problem:
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    1. take full backup with --lock-ddl=reduced

    2. create table t1(a INT), lets say space_id 10

    3. start incremental backup and pause before backup_start() function (we take Bakcup lock here)

    4. incremental backup copied t1.ibd.meta and t1.ibd.delta by this time

    5. DROP TABLE t1

    6. resume the incremental backup. 10.del file is created

    7. prepare the full backup with --apply-log-only

    8. prepare incremental backup

    incremental backup prepare first processes the .del files. before this all tabelspaces are loaded via .ibd scan

    since there is no t1.ibd in backup directory( it is only present as meta and delta file) in incremental backup directory, space_id with 10 is not in cache.

    Hence prepare_handle_del_files() will not delete the files related to space_id 10.
    We end up with orphan .ibd or .ibu files. Server ignore orphan .ibd
    But if the tablespace is undo tablespace, orphan .ibu are not ignored by server.
    Server discovers them via *.ibu scan. This can lead to assertion failures.