Implement dynamic per arena control over dirty page purging.

Add mallctls:
- arenas.lg_dirty_mult is initialized via opt.lg_dirty_mult, and can be
  modified to change the initial lg_dirty_mult setting for newly created
  arenas.
- arena.<i>.lg_dirty_mult controls an individual arena's dirty page
  purging threshold, and synchronously triggers any purging that may be
  necessary to maintain the constraint.
- arena.<i>.chunk.purge allows the per arena dirty page purging function
  to be replaced.

This resolves #93.

src/chunk.c

@@ -391,8 +391,10 @@ chunk_record(arena_t *arena, extent_tree_t *chunks_szad,
 			 * pages have already been purged, so that this is only
 			 * a virtual memory leak.
 			 */
-			if (cache)
-				pages_purge(chunk, size);
+			if (cache) {
+				chunk_purge_wrapper(arena, arena->chunk_purge,
+				    chunk, 0, size);
+			}
 			goto label_return;
 		}
 		extent_node_init(node, arena, chunk, size, !unzeroed);
@@ -485,6 +487,37 @@ chunk_dalloc_wrapper(arena_t *arena, chunk_dalloc_t *chunk_dalloc, void *chunk,
 		JEMALLOC_VALGRIND_MAKE_MEM_NOACCESS(chunk, size);
 }
 
+bool
+chunk_purge_arena(arena_t *arena, void *chunk, size_t offset, size_t length)
+{
+
+	assert(chunk != NULL);
+	assert(CHUNK_ADDR2BASE(chunk) == chunk);
+	assert((offset & PAGE_MASK) == 0);
+	assert(length != 0);
+	assert((length & PAGE_MASK) == 0);
+
+	return (pages_purge((void *)((uintptr_t)chunk + (uintptr_t)offset),
+	    length));
+}
+
+bool
+chunk_purge_default(void *chunk, size_t offset, size_t length,
+    unsigned arena_ind)
+{
+
+	return (chunk_purge_arena(chunk_arena_get(arena_ind), chunk, offset,
+	    length));
+}
+
+bool
+chunk_purge_wrapper(arena_t *arena, chunk_purge_t *chunk_purge, void *chunk,
+    size_t offset, size_t length)
+{
+
+	return (chunk_purge(chunk, offset, length, arena->ind));
+}
+
 static rtree_node_elm_t *
 chunks_rtree_node_alloc(size_t nelms)
 {