Adding rec_lsn to Maria's page cache. Misc fixes to Checkpoint.


mysys/mf_pagecache.c:
  adding rec_lsn, the LSN when a page first became dirty.
  It is set when unlocking a page (TODO: should also be set when
  the unlocking is an implicit part of pagecache_write()).
  It is reset in link_to_file_list() and free_block()
  (one of which is used every time we flush a block).
  It is a ulonglong and not LSN, because its destination is comparisons
  for which ulonglong is better than a struct.
storage/maria/ma_checkpoint.c:
  misc fixes to Checkpoint (updates now that the transaction manager
  and the page cache are more known)
storage/maria/ma_close.c:
  an important note for the future.
storage/maria/ma_least_recently_dirtied.c:
  comment