WL#5872 "avoid using global heap memory to remember autoincrement values for statement-based binlog".
As long as WL3404 "new binlog event Insert_id_list_event to store multiple auto_increment intervals"
is not implemented, Discrete_intervals_list needs to contain only the first interval,
so it makes sense to store this interval as a member of Discrete_intervals_list,
rather than allocating it on the global heap with new(). This is to save one new()
per statement inserting into an auto-increment column, and is a sub-part of
WL5774 "Decrease number of malloc's for normal DML queries".
Even when WL3404 is implemented, it will still make sense to avoid new()
for the first interval, as we expect one-interval lists to be common.
Patch includes parts by Sven Sandberg.