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.