MDEV-14425 improved write performance less than it was hoped. We are still seeing log_sys.mutex and buf_pool.mutex dominating the wait times.

A prominent bottleneck was identified to be log_sys.mutex between log_sys.append_prepare() and log_close(). To protect the copying of data to log_sys.buf, we only need a shared log_sys.latch, which can replace log_sys.mutex and log_sys.flush_order_mutex. The log_sys.lsn and the position of log_sys.buf needs to be protected by a new log_sys.lsn_lock. Writes of log_sys.buf to the file system will have to acquire exclusive log_sys.latch.

While working on this code, we can also make mtr_t::commit() invoke buf_pool.page_cleaner_wakeup() and update Innodb_buffer_pool_write_requests at most once per call.