Preparation: for the two-checkpoint rule we will flush all pages
which have block->rec_lsn <= last_checkpoint_lsn. Pages with
rec_lsn not yet set (i.e. ==0) were dirtied very recently and so should
not be flushed (otherwise it's not efficient: we should try to let
a page be modified several times in memory before we flush it to disk).
To make this easy, "block->rec_lsn not yet set" is now expressed
with block->rec_lsn==LSN_MAX, not block->rec_lsn==0 anymore.
It is easier this way because LSN_MAX>last_checkpoint_lsn whereas
0<=last_checkpoint_lsn.


storage/maria/ma_blockrec.c:
  typo
storage/maria/ma_loghandler.c:
  typo
storage/maria/ma_loghandler_lsn.h:
  LSN_MAX
storage/maria/ma_pagecache.c:
  "block->rec_lsn not yet set" is now expressed by block->rec_lsn==LSN_MAX,
  not block_rec_lsn==0.