- cleanups, simplifications
- moving the construction of the "dirty pages table" into the
pagecache where it belongs (because it's the pagecache which knows
dirty pages). TODO: do the same soon for the "transactions table".
- fix for a small bug in the pagecache (decrementation of "changed_blocks")


include/pagecache.h:
  prototype
mysys/mf_pagecache.c:
  m_string.h moves up for LEX_STRING to be known for pagecache.h.
  In pagecache_delete_page(), we must decrement "blocks_changed" even
  if we just delete the page without flushing it.
  A new function pagecache_collect_changed_blocks_with_LSN()
  (used by the Checkpoint module), which stores information about the
  changed blocks (a.k.a. "the dirty pages table") into a LEX_STRING.
  This function is not tested now, it will be when there is a Checkpoint.
storage/maria/ma_checkpoint.c:
  refining the checkpoint code: factoring functions, moving the
  construction of the "dirty pages table" into mf_pagecache.c
  (I'll do the same with the construction of the "transactions table"
  once Serg tells me what's the best way to do it).
storage/maria/ma_least_recently_dirtied.c:
  Simplifying the thread which does background flushing of
  least-recently-dirtied pages:
  - in first version that thread will not flush, just do checkpoints
  - in 2nd version, flushing should re-use existing page cache functions
  like flush_pagecache_blocks().
unittest/mysys/test_file.h:
  m_string.h moves up for LEX_STRING to be known in pagecache.h