we now make a distinction between if the master is < 3.23.57, 3.23 && >=57, and 4.x
(before the 2 3.23 were one). This is because in 3.23.57 we have a way to distinguish between
a Start_log_event written at server startup and one written at FLUSH LOGS, so we
have a way to know if the slave must drop old temp tables or not.
Change: mi->old_format was bool, now it's enum (to handle 3 cases). However, functions
which had 'bool old_format' as an argument have their prototypes unchanged, because
the old old_format == 0 now corresponds to the enum value BINLOG_FORMAT_CURRENT which
is equal to 0, so boolean tests are left untouched. The only case were we use mi->old_format
as an enum instead of casting it implicitly to a bool, is in Start_log_event::exec_event,
where we want to distinguish between the 3 possible enum values.