Modify dirty page purging algorithm.

Convert chunks_dirty from a red-black tree to a doubly linked list,
and use it to purge dirty pages from chunks in FIFO order.

Add a lock around the code that purges dirty pages via madvise(2), in
order to avoid kernel contention.  If lock acquisition fails,
indefinitely postpone purging dirty pages.

Add a lower limit of one chunk worth of dirty pages per arena for
purging, in addition to the active:dirty ratio.

When purging, purge all dirty pages from at least one chunk, but rather
than purging enough pages to drop to half the purging threshold, merely
drop to the threshold.