- Comments for future devs.
- Start_log_event::exec_event() : when we hit it, do a rollback.
- We don't need LOG_EVENT_FORCED_ROTATE_F.
- Stop_log_event::exec_event() : when we hit it, we needn't clean anything.
- Removed LOG_EVENT_TIME_F and LOG_EVENT_FORCED_ROTATE_F.
- We don't need Stop events in the relay log.
- Now filtering of server id is done in the I/O thread first.


sql/log.cc:
  We don't need LOG_EVENT_FORCED_ROTATE_F
sql/log_event.cc:
  - Comments for future devs.
  - Start_log_event::exec_event() : when we hit it, do a rollback. If the SQL
  thread was inside a transaction (for example, the master died while writing
  to the binlog, so did not commit (because we write to the binlog before committing),
  so will rollback), it's sensible to rollback. If we're not in a transaction,
  rollback will not hurt (it will do nothing).
  - We don't need LOG_EVENT_FORCED_ROTATE_F.
  - Stop_log_event::exec_event() : when we hit it, we needn't clean anything, because
  each threads has already written some cleaning statements (DROP TEMPORARY TABLE,
  DO RELEASE_LOCK); we still clean in Start_log_event::exec_event() (if 4.x).
sql/log_event.h:
  - Comments for future devs.
  - Removed LOG_EVENT_TIME_F and LOG_EVENT_FORCED_ROTATE_F.
sql/slave.cc:
  - We don't need Stop events in the relay log (see changeset's description about
  log_event.cc). So we can make event queuing (writing to the relay log) simpler.
  - Something that was marked TODO: now filtering of server id (if the server id
  of this event is the same as mine ignore it) is done in the I/O thread if
  the master is 4.x (it still also done in the SQL thread whatever the version of
  the master is, for safe upgrades). This saves disk space.
sql/slave.h:
  We don't need master_info.ignore_stop_event anymore, as we don't write Stop_event
  to the relay log anymore.