The recent commit 4ca355d8 (MDEV-33894)
caused a serious regression for online InnoDB ib_logfile0 resizing,
breaking crash-safety unless the memory-mapped log file interface is
being used. However, the log resizing was broken also before this.

To prevent such regressions in the future, we extend the test
innodb.log_file_size_online with a kill and restart of the server
and with some writes running concurrently with the log size change.
When run enough many times, this test revealed all the bugs that
are being fixed by the code changes.

log_t::resize_start(): Do not allow the resized log to start before
the current log sequence number. In this way, there is no need to
copy anything to the first block of resize_buf. The previous logic
regarding that was incorrect in two ways. First, we would have to
copy from the last written buffer (buf or flush_buf). Second, we failed
to ensure that the mini-transaction end marker bytes would be 1
in the buffer. If the source ib_logfile0 had wrapped around an odd number
of times, the end marker would be 0. This was occasionally observed
when running the test innodb.log_file_size_online.

log_t::resize_write_buf(): To adjust for the resize_start() change,
do not write anything that would be before the resize_lsn.
Take the buffer (resize_buf or resize_flush_buf) as a parameter.
Starting with commit 4ca355d8
we no longer swap buffers when rewriting the last log block.

log_t::append(): Define as a static function; only some debug
assertions need to refer to the log_sys object.

innodb_log_file_size_update(): Wake up the buf_flush_page_cleaner()
if needed, and wait for it to complete a batch while waiting for
the log resizing to be completed. If the current LSN is behind the
resize target LSN, we will write redundant FILE_CHECKPOINT records to
ensure that the log resizing completes. If the buf_pool.flush_list is
empty or the buf_flush_page_cleaner() is stuck for some reason, our wait
will time out in 5 seconds, so that we can periodically check if the
execution of SET GLOBAL innodb_log_file_size was aborted. Previously,
we could get into a busy loop here while the buf_flush_page_cleaner()
would remain idle.