- Before this patch during startup all slave threads was started without
  any check that they had started properly.
- If one did a START SLAVE, STOP SLAVE or CHANGE MASTER as first command to the server
  there was a chance that server could access structures that where not
  properly  initialized which could lead to crashes in
  Log_event::read_log_event
- Fixed by waiting for slave threads to start up properly also during
  server startup, like we do with START SLAVE.