btr_defragment_thread(): Create the thread in the same place as other
threads. Do not invoke btr_defragment_shutdown(), because
row_drop_tables_for_mysql_in_background() in the master thread can still
keep invoking btr_defragment_remove_table().
logs_empty_and_mark_files_at_shutdown(): Wait for btr_defragment_thread()
to exit.
innobase_start_or_create_for_mysql(), innobase_shutdown_for_mysql():
Skip encryption and scrubbing in innodb_read_only_mode.
srv_export_innodb_status(): Do not export encryption or scrubbing
statistics in innodb_read_only mode, because the threads will not
be running.
The InnoDB source code contains quite a few references to a closed-source
hot backup tool which was originally called InnoDB Hot Backup (ibbackup)
and later incorporated in MySQL Enterprise Backup.
The open source backup tool XtraBackup uses the full database for recovery.
So, the references to UNIV_HOTBACKUP are only cluttering the source code.
