The log_sys.lsn_lock that was introduced in
commit a635c406
had better be located in the same cache line with log_sys.latch
so that log_t::append_prepare() needs to modify only two first
cache lines where log_sys is stored.

log_t::lsn_lock: On Linux, change the type from pthread_mutex_t to
something that may be as small as 32 bits, to pack more data members
in the same cache line. On Microsoft Windows, CRITICAL_SECTION works
better.

log_t::check_flush_or_checkpoint_: Renamed to need_checkpoint.
There is no need to pause all writer threads in log_free_check() when
we only need to write log_sys.buf to ib_logfile0. That will be done in
mtr_t::commit().

log_t::append_prepare_wait(): Make the member function non-static
to simplify the call interface, and add a parameter for the LSN.

log_t::append_prepare(): Invoke append_prepare_wait() at most once.
Only set_check_for_checkpoint() if a log checkpoint needs to
be written. If the log buffer needs to be written, we will take care
of it ourselves later in our caller. This will reduce interference
with log_free_check() in other threads.

mtr_t::commit(): Call log_write_up_to() if needed.

log_t::get_write_target(): Return a log_write_up_to() target
to mtr_t::commit().

buf_flush_ahead(): If we are in furious flushing, call
log_sys.set_check_for_checkpoint() so that all writers will wait
in log_free_check() until the checkpoint is done. Otherwise,
the test innodb.insert_into_empty could occasionally report
an error "Crash recovery is broken".

log_check_margins(): Replaced by log_free_check().

log_flush_margin(): Removed. This is part of mtr_t::commit()
and other operations that write log.

log_t::create(), log_t::attach(): Guarantee that buf_free < max_buf_free
will always hold on PMEM, to satisfy an assumption of
log_t::get_write_target().

log_write_up_to(): Assert lsn!=0. Such calls are not incorrect, but it
is cheaper to test that single unlikely condition in mtr_t::commit()
rather than test several conditions in log_write_up_to().

innodb_drop_database(), unlock_and_close_files(): Check the LSN before
calling log_write_up_to().

ha_innobase::commit_inplace_alter_table(): Remove redundant calls to
log_write_up_to() after calling unlock_and_close_files().

Reviewed by: Vladislav Vaintroub
Stress tested by: Matthias Leich
Performance tested by: Steve Shaw