don't take a checkpoint at end of REDO phase because blocks' rec_lsn
are not set. Fixes some assertion failures during tests of killing
the UNDO phase (recovery then started from the end-of-REDO-phase
checkpoint, wrongly ignoring REDOs before that record).


storage/maria/ma_blockrec.c:
  note for a bug (mail sent to colleagues, put on todo)
storage/maria/ma_recovery.c:
  this checkpoint at end of REDO phase is incorrect, disabling it.
  The one at end of UNDO phase remains.