Fixing bad comments (I remember my maths' teacher "one late night you'll
obey to the simplifications made by your tired neurons"; exactly
what happened here). In Checkpoint, when we flush a table's state
we must flush all log records (WAL), not only those before checkpoint
started.


storage/maria/ma_bitmap.c:
  there was a flaw in reasoning, bug does exist.
storage/maria/ma_blockrec.c:
  moving piece of comment to ma_checkpoint.c
storage/maria/ma_checkpoint.c:
  Comments.
  When checkpoint flushes a state, WAL imposes that all records up
  to this state have been flushed, not only up to checkpoint_start_log_horizon.
storage/maria/ma_recovery.c:
  finishing comment.