- serializing calls to flush_pagecache_blocks_int() on the same file
to avoid known concurrency bugs
- having that, we can now enable the background thread, as the
flushes it does are now supposedly safe in concurrent situations.
- new type of flush FLUSH_KEEP_LAZY: when the background checkpoint
thread is flushing a packet of dirty pages between two checkpoints,
it uses this flush type, indeed if a file is already being flushed
by another thread it's smarter to move on to the next file than wait.
- maria_checkpoint_frequency renamed to maria_checkpoint_interval.


include/my_sys.h:
  new type of flushing for the page cache: FLUSH_KEEP_LAZY
mysql-test/r/maria.result:
  result update
mysys/mf_keycache.c:
  indentation. No FLUSH_KEEP_LAZY support in key cache.
storage/maria/ha_maria.cc:
  maria_checkpoint_frequency was somehow a hidden part of the
  Checkpoint API and that was not good. Now we have checkpoint_interval,
  local to ha_maria.cc, which serves as container for the user-visible
  maria_checkpoint_interval global variable; setting it calls
  update_checkpoint_interval which passes the new value to
  ma_checkpoint_init(). There is no hiding anymore.
  By default, enable background thread which does checkpoints
  every 30 seconds, and dirty page flush in between. That thread takes
  a checkpoint when it ends, so no need for maria_hton_panic to take one.
  The | is | and not ||, because maria_panic() must always be called.
  frequency->interval.
storage/maria/ma_checkpoint.c:
  Use FLUSH_KEEP_LAZY for background thread when it flushes packets of
  dirty pages between two checkpoints: it is smarter to move on to
  the next file than wait for it to have been completely flushed, which
  may take long.
  Comments about flush concurrency bugs moved from ma_pagecache.c.
  Removing out-of-date comment.
  frequency->interval.
  create_background_thread -> (interval>0).
  In ma_checkpoint_background(), some variables need to be preserved
  between iterations.
storage/maria/ma_checkpoint.h:
  new prototype
storage/maria/ma_pagecache.c:
  - concurrent calls of flush_pagecache_blocks_int() on the same file
  cause bugs (see @note in that function); we fix them by serializing
  in this situation. For that we use a global hash of (file, wqueue).
  When flush_pagecache_blocks_int() starts it looks into the hash,
  using the file as key. If not found, it inserts (file,wqueue) into the
  hash, flushes the file, and finally removes itself from the hash and
  wakes up any waiter in the queue. If found, it adds itself to the
  wqueue and waits.
  - As a by-product, we can remove changed_blocks_is_incomplete
  and replace it by scanning the hash, replace the sleep() by a queue wait.
  - new type of flush FLUSH_KEEP_LAZY: when flushing a file, if it's
  already being flushed by another thread (even partially), return
  immediately.
storage/maria/ma_pagecache.h:
  In pagecache, a hash of files currently being flushed (i.e. there
  is a call to flush_pagecache_blocks_int() for them).
storage/maria/ma_recovery.c:
  new prototype
storage/maria/ma_test1.c:
  new prototype
storage/maria/ma_test2.c:
  new prototype