Vladislav Vaintroub suggested that invoking log_flush_up_to()
for every page could perform better than invoking a log write
between buf_pool.flush_list batches, like we started doing in
commit 3a9a3be1 (MDEV-23855).
This could depend on the sequence in which pages are being
modified. The buf_pool.flush_list is ordered by
oldest_modification, while the FIL_PAGE_LSN of the pages is
theoretically independent of that. In the pathological case,
we will wait for a log write before writing each individual page.

It turns out that we can defer the call to log_flush_up_to()
until just before submitting the page write. If the doublewrite
buffer is being used, we can submit a write batch of "future" pages
to the doublewrite buffer, and only wait for the log write right
before we are writing an already doublewritten page.
The next doublewrite batch will not be initiated before the last
page write from the current batch has completed.

When a future version introduces asynchronous writes if the log,
we could initiate a write at the start of a flushing batch, to
reduce waiting further.