For MDEV-14481, we must defer the change buffer merge to the moment when the secondary index leaf page is requested by a user thread.
This would also simplify MDEV-16526, because the change buffer I/O would not have to be treated as a special case.
The change buffer format will not be changed as part of this task. That could remain part of MDEV-11634.
As part of this, the counter innodb_ibuf_merge_usec will be removed from information_schema.innodb_metrics.