With MDEV-15132 in MariaDB 10.3.5, InnoDB no longer writes the
transaction identifier to the TRX_SYS page. The information is
only written to undo log headers and sometimes rollback segment
headers. Because the setting innodb_force_recovery=5 will skip
reading any of those pages, the maximum transaction identifier
will no longer be determined.

innobase_map_isolation_level(): Always report READ UNCOMMITTED
if innodb_force_recovery has been set to 5 or more, or
innodb_read_only is set. This will avoid errors reported by
lock_check_trx_id_sanity() and ReadView::check_trx_id_sanity().

lock_clust_rec_cons_read_sees(): Do not check for
innodb_read_only, now that innobase_map_isolation_level() will
guarantee that no read view will be created or used.

row_search_mvcc(): Do not check for innodb_force_recovery<5,
now that innobase_map_isolation_level() will guarantee that
no read view will be created or used.