consolidate server transactional logs

Intro

The server has binary log, where transactions are recorded, a mmap-based "tc-log" for internal 2PC support (when binlog is disabled) and a ddl log for atomic partitioning DDLs. This latter log cannot be used as-is to make all DDLs atomic, so there's a chance there will be a third one.

This is three logs too many. The task is to remove partitioning ddl log and tc-log and use binlog for everything.

Benefits

• only one implementation of logging
  • already supports group commits, rotation, purging
  • binlog-based crash recovery is already implemented and works
  • intensively used, so much better tested and trustworthy than the others
• only one file to sync
• only one file to perform recovery from

Drawbacks

• binlog will contain events irrelevant for replication or backups
• binlog cannot be turned off

Implementation considerations

• Binlog cannot be turned off, so "binary logging disabled" will mean that DMLs will not be binlogged and only recovery-relevant events will be. One consequence of this is that we can remove all testing of 'binlog-is-open' in the code.
• mysql_binlog_send(), probably, should filter out recovery events and not send them to slaves. Alternatively, it'll send everything and slaves ignore them
• It's better to have just one new binlog event "DDL log" and sub-events in it. This would make it easier to filter it out and we avoid spamming binlog/replication with tens of irrelevant new event types.