innodb_log_archive=ON file creation causes log_sys.latch contention

I made a questionable design choice in MDEV-37949 in order to simplify the initial implementation of the innodb_log_archive=ON format: When a new ib_*.log file is being created and extended to innodb_log_file_size, we will hold log_sys.latch in exclusive mode. This will create significant contention to other concurrent threads.

A better choice would be to preallocate a spare file while not holding any mutex. The most natural place for that could be the buf_flush_page_cleaner() thread.

Creating a file upfront might also allow more graceful handling of out-of-space scenarios. Currently, we intentionally crash the server if we fail to create a log file. If we preallocate a log file, in some cases under moderate write load there could be enough time to gracefully switch to read-only operation. Because this is tricky to test and implement, I would leave this out of the scope of the fix for now.