Integrate whole chunks into unused dirty page purging machinery.

Extend per arena unused dirty page purging to manage unused dirty chunks
in aaddtion to unused dirty runs.  Rather than immediately unmapping
deallocated chunks (or purging them in the --disable-munmap case), store
them in a separate set of trees, chunks_[sz]ad_dirty.  Preferrentially
allocate dirty chunks.  When excessive unused dirty pages accumulate,
purge runs and chunks in ingegrated LRU order (and unmap chunks in the
--enable-munmap case).

Refactor extent_node_t to provide accessor functions.

include/jemalloc/internal/chunk.h

@@ -44,8 +44,10 @@ void	*chunk_alloc_arena(chunk_alloc_t *chunk_alloc,
     size_t size, size_t alignment, bool *zero);
 void	*chunk_alloc_default(void *new_addr, size_t size, size_t alignment,
     bool *zero, unsigned arena_ind);
-void	chunk_unmap(arena_t *arena, void *chunk, size_t size);
+void	chunk_record(arena_t *arena, extent_tree_t *chunks_szad,
+    extent_tree_t *chunks_ad, bool dirty, void *chunk, size_t size);
 bool	chunk_dalloc_default(void *chunk, size_t size, unsigned arena_ind);
+void	chunk_unmap(arena_t *arena, void *chunk, size_t size);
 bool	chunk_boot(void);
 void	chunk_prefork(void);
 void	chunk_postfork_parent(void);