in write_changed_bitmap(), and page cache forbids that. Here we make the page
cache more relaxed. Original patch by Sanja, simplified by me as limited to
not-locked. See comment of ma_bitmap.c.
With that, maria_stress.yy runs until hitting BUG 39665.

storage/maria/ma_bitmap.c:
  A thread which unpins bitmap pages in _ma_bitmap_unpin_all() sometimes
  hit an assertion in the page cache (info!=0 in remove_pin()) which states
  that you can unpin/unlock only what *you* have pinned/locked.
  Fixed by setting the new last parameter of pagecache_unlock_by_link()
  to TRUE in _ma_bitmap_unpin_all().
storage/maria/ma_blockrec.c:
  new prototype and splitting assertion in three (3rd one fires: BUG 39665)
storage/maria/ma_check.c:
  new prototype
storage/maria/ma_key_recover.c:
  new prototype
storage/maria/ma_loghandler.c:
  new prototype
storage/maria/ma_pagecache.c:
  Allow a thread to unpin, with pagecache_unlock_by_link(), a non-locked page pinned by others.
  This is a hack for _ma_bitmap_unpin_all() which needs to unpin pages which were
  pinned by other threads in write_changed_bitmap().
storage/maria/ma_pagecache.h:
  new prototype
storage/maria/ma_preload.c:
  new prototype
storage/maria/unittest/ma_pagecache_rwconsist.c:
  new prototype
storage/maria/unittest/ma_pagecache_single.c:
  new prototype