As pointed out by Andrei Elkin, the previous fix did not fix one
race condition that may have caused the observed hang.

innodb_log_flush_request(): If we are enqueueing the very first
request at the same time the log write is being completed,
we must ensure that a near-concurrent call to log_flush_notify()
will not result in a missed notification. We guarantee this by
release-acquire operations on log_requests.start and
log_sys.flushed_to_disk_lsn.

log_flush_notify_and_unlock(): Cleanup: Always release the mutex.

log_sys_t::get_flushed_lsn(): Use acquire memory order.

log_sys_t::set_flushed_lsn(): Use release memory order.

log_sys_t::set_lsn(): Use release memory order.

log_sys_t::get_lsn(): Use relaxed memory order by default, and
allow the caller to specify acquire memory order explicitly.
Whenever the log_sys.mutex is being held or when log writes are
prohibited during startup, we can use a relaxed load. Likewise,
in some assertions where reading a stale value of log_sys.lsn
should not matter, we can use a relaxed load.

This will cause some additional instructions to be emitted on
architectures that do not implement Total Store Ordering (TSO),
such as POWER, ARM, and RISC-V Weak Memory Ordering (RVWMO).