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.
This should also allow us to remove the buf_pool->watch and BUF_BLOCK_POOL_WATCH.
The change buffer format will not be changed as part of this task. That could remain part of MDEV-11634.