Ability for flush_pagecache_blocks() to flush only certain pages of
a file, as instructed by an option "filter" pointer-to-function argument;
Checkpoint and background dirty page flushing use that to flush only
pages which have been dirty for long enough and bitmap pages.
Fix for a bug in flush_cached_blocks() (no idea if it could produce
a bug in real life, but theoretically it is).
Testing checkpoint in ma_test_recovery via ma_test1 and ma_test2.
Background checkpoint & dirty pages flush thread is still disabled
by default in ha_maria.


mysql-test/r/maria.result:
  result update
storage/maria/ha_maria.cc:
  blank after function comment
storage/maria/ma_checkpoint.c:
  Using an enum instead of 0/1/2 (applying Sanja's review comments).
  The comment about "this is an horizon" can be removed as Sanja
  created translog_next_LSN() which parse_checkpoint_record() uses.
  Variables in ma_checkpoint_background() cannot be declared in the
  for() as their value must not be reset at each iteration!
storage/maria/ma_pagecache.c:
  adding to flush_pagecache_blocks() optional arguments 'filter'
  (pointer to function) and 'filter_arg'; if filter!=NULL this function
  will be called for each block of the file and will reply if this
  block and following ones should be flushed or not (3 possible
  replies).
  Fixing a bug when flush_cached_blocks() skips a pinned page: it has
  to unset PCBLOCK_IN_FLUSH set by flush_pagecache_blocks_int().
storage/maria/ma_pagecache.h:
  flush_pagecache_blocks() is changed to take "filter" and "filter_arg"
  arguments. "filter", if it is not NULL, may return one value
  among enum pagecache_flush_filter_result.
storage/maria/ma_recovery.c:
  open_count=0 when closing tables at the end of recovery.
storage/maria/ma_test1.c:
  Optional checkpoints (-H#) at various stages (stages similar
  to --testflag), for testing of checkpoints.
storage/maria/ma_test2.c:
  Optional checkpoints (-H#) at various stages (stages similar
  to -t), for testing of checkpoints.
storage/maria/ma_test_recovery.expected:
  Result update: the results of the additional test run with -H#
  (checkpoints) are added here. They are exactly identical to without
  checkpoints except that the index's Root (printed by maria_chk)
  is more correct when using checkpoints. This is because checkpoint
  flushed the state, so it happens to be correct, while no-checkpoint
  does not flush the state, and recovery does not recover indexes
  so Root is never fixed. When we recover indices, this will go away.
storage/maria/ma_test_recovery:
  We duplicate the loop of tests to add an additional run with
  checkpoints at various stages, to see if maria_read_log
  uses them fine.