Introduce InnoDB monitor for last tracked LSN

Introduce new InnoDB monitor log_lsn_tracked, whose value is the last tracked LSN. Its value is 0 if tracking is not enabled.

Starting with 8.0.16, MTR has been changed that replace_regex no longer works on backtick query result substitution on variable assignment. This breaks innodb.percona_changed_page_bmp_flush_5446. Instead of rewriting SHOW ENGINE INNODB STATUS parser yet another time, introducing a new monitor will allow for simpler testcase while being useful by itself.