already supports pin-without-lock so implementation of this WL is instant and
done here. This could improve concurrency. No testcase, this requires
multiple threads and is automatically tested at push time by maria_stress.yy (pushbuild2).

storage/maria/ma_bitmap.c:
  As the page cache supports pinning without write-locking, we don't take write lock
  in write_changed_bitmap(), only a pin; this could improve concurrency (WL#4595).