Part of MDEV-13073 AliSQL Optimize performance of semisync

The idea it to use a dedicated lock detecting if there is new data in
the master's binary log instead of the overused LOCK_log.

Changes:
- Use dedicated COND variables for the relay and binary log signaling.
  This was needed as we where the old 'update_cond' variable was used
  with different mutex's, which could cause deadlocks.
   - Relay log uses now COND_relay_log_updated and LOCK_log
   - Binary log uses now COND_bin_log_updated and LOCK_binlog_end_pos
- Renamed signal_cnt to relay_signal_cnt (as we now have two signals)
- Added some missing error handling in MYSQL_BIN_LOG::new_file_impl()
- Reformatted some comments with old style
- Renamed m_key_LOCK_binlog_end_pos to key_LOCK_binlog_end_pos
- Changed 'signal_update()' to update_binlog_end_pos() which works for
  both relay and binary log