Log MDL state transitions. Trace-friendly message
format. DBUG_LOCK_FILE replaced by thread-local storage.

Logged states legend:
  Seized   lock was acquired without waiting
  Waiting  lock is waiting
  Acquired lock was acquired after waiting
  Released lock was released
  Deadlock lock was aborted due to deadlock
  Timeout  lock was aborted due to timeout >0
  Nowait   lock was aborted due to zero timeout
  Killed   lock was aborted due to kill message
  OOM	   can not acquire because out of memory

Usage:
  mtr --mysqld=--debug=d,mdl,query:i:o,/tmp/mdl.log

Cleanup from garbage messages:
  sed -i -re \
  '/(mysql|performance_schema|sys|mtr)\// d; /MDL_BACKUP_/ d' \
  /tmp/mdl.log